Convert README format from reStructuredText to markdown (#357)

Markdown is much easier to manage and more GitHub compatible.
Also updated the badges links and removed deprecated ones.

Author: mxamin

.appveyor.yml

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+# python-xmlsec
+
+[![image](https://img.shields.io/pypi/v/xmlsec.svg?logo=python&logoColor=white)](https://pypi.python.org/pypi/xmlsec)
+[![pre-commit.ci status](https://results.pre-commit.ci/badge/github/xmlsec/python-xmlsec/master.svg)](https://results.pre-commit.ci/latest/github/xmlsec/python-xmlsec/master)
+[![image](https://github.com/xmlsec/python-xmlsec/actions/workflows/manylinux.yml/badge.svg)](https://github.com/xmlsec/python-xmlsec/actions/workflows/manylinux.yml)
+[![image](https://github.com/xmlsec/python-xmlsec/actions/workflows/macosx.yml/badge.svg)](https://github.com/xmlsec/python-xmlsec/actions/workflows/macosx.yml)
+[![image](https://github.com/xmlsec/python-xmlsec/actions/workflows/linuxbrew.yml/badge.svg)](https://github.com/xmlsec/python-xmlsec/actions/workflows/linuxbrew.yml)
+[![image](https://codecov.io/gh/xmlsec/python-xmlsec/branch/master/graph/badge.svg)](https://codecov.io/gh/xmlsec/python-xmlsec)
+[![Documentation Status](https://img.shields.io/readthedocs/xmlsec/latest?logo=read-the-docs)](https://xmlsec.readthedocs.io/en/latest/?badge=latest)
+
+Python bindings for the [XML Security
+Library](https://www.aleksey.com/xmlsec/).
+
+## Documentation
+
+Documentation for `xmlsec` can be found at
+[xmlsec.readthedocs.io](https://xmlsec.readthedocs.io/).
+
+## Usage
+
+Check the
+[examples](https://xmlsec.readthedocs.io/en/latest/examples.html)
+section in the documentation to see various examples of signing and
+verifying using the library.
+
+## Requirements
+
+- `libxml2 >= 2.9.1`
+- `libxmlsec1 >= 1.2.33`
+
+## Install
+
+`xmlsec` is available on PyPI:
+
+``` bash
+pip install xmlsec
+```
+
+Depending on your OS, you may need to install the required native
+libraries first:
+
+### Linux (Debian)
+
+``` bash
+apt-get install pkg-config libxml2-dev libxmlsec1-dev libxmlsec1-openssl
+```
+
+Note: There is no required version of LibXML2 for Ubuntu Precise, so you
+need to download and install it manually.
+
+``` bash
+wget http://xmlsoft.org/sources/libxml2-2.9.1.tar.gz
+tar -xvf libxml2-2.9.1.tar.gz
+cd libxml2-2.9.1
+./configure && make && make install
+```
+
+### Linux (CentOS)
+
+``` bash
+yum install libxml2-devel xmlsec1-devel xmlsec1-openssl-devel libtool-ltdl-devel
+```
+
+### Linux (Fedora)
+
+``` bash
+dnf install libxml2-devel xmlsec1-devel xmlsec1-openssl-devel libtool-ltdl-devel
+```
+
+### Mac
+
+``` bash
+brew install libxml2 libxmlsec1 pkg-config
+```
+
+or
+
+``` bash
+port install libxml2 xmlsec pkgconfig
+```
+
+### Alpine
+
+``` bash
+apk add build-base libressl libffi-dev libressl-dev libxslt-dev libxml2-dev xmlsec-dev xmlsec
+```
+
+## Troubleshooting
+
+### Mac
+
+If you get any fatal errors about missing `.h` files, update your
+`C_INCLUDE_PATH` environment variable to include the appropriate files
+from the `libxml2` and `libxmlsec1` libraries.
+
+### Windows
+
+Starting with 1.3.7, prebuilt wheels are available for Windows, so
+running `pip install xmlsec` should suffice. If you want to build from
+source:
+
+1. Configure build environment, see
+   [wiki.python.org](https://wiki.python.org/moin/WindowsCompilers) for
+   more details.
+
+2. Install from source dist:
+
+    ``` bash
+    pip install xmlsec --no-binary=xmlsec
+    ```
+
+## Building from source
+
+1. Clone the `xmlsec` source code repository to your local computer.
+
+    ``` bash
+    git clone https://github.com/xmlsec/python-xmlsec.git
+    ```
+
+2. Change into the `python-xmlsec` root directory.
+
+    ``` bash
+    cd /path/to/xmlsec
+    ```
+
+3. Install the project and all its dependencies using `pip`.
+
+   ``` bash
+   pip install .
+   ```
+
+## Contributing
+
+### Setting up your environment
+
+1. Follow steps 1 and 2 of the [manual installation
+   instructions](#building-from-source).
+
+2. Initialize a virtual environment to develop in. This is done so as
+   to ensure every contributor is working with close-to-identical
+   versions of packages.
+
+   ``` bash
+   mkvirtualenv xmlsec
+   ```
+
+   The `mkvirtualenv` command is available from `virtualenvwrapper`
+   package which can be installed by following
+   [link](http://virtualenvwrapper.readthedocs.org/en/latest/install.html#basic-installation).
+
+3. Activate the created virtual environment:
+
+   ``` bash
+   workon xmlsec
+   ```
+
+4. Install `xmlsec` in development mode with testing enabled. This will
+   download all dependencies required for running the unit tests.
+
+   ``` bash
+   pip install -r requirements-test.txt
+   pip install -e "."
+   ```
+
+### Running the test suite
+
+1. [Set up your environment](#setting-up-your-environment).
+
+2. Run the unit tests.
+
+   ``` bash
+   pytest tests
+   ```
+
+3. Tests configuration
+
+   Env variable `PYXMLSEC_TEST_ITERATIONS` specifies number of test
+   iterations to detect memory leaks.
+
+### Reporting an issue
+
+Please attach the output of following information:
+
+- version of `xmlsec`
+- version of `libxmlsec1`
+- version of `libxml2`
+- output from the command
+
+  ``` bash
+  pkg-config --cflags xmlsec1
+  ```
+
+## License
+
+Unless otherwise noted, all files contained within this project are
+licensed under the MIT open source license. See the included `LICENSE`
+file or visit [opensource.org](http://opensource.org/licenses/MIT) for
+more information.