Document best practices on installing third party libraries that are not part of the standard Python library

[fdrozdowski]

We should document the best practices on how to add third party libraries to plugin's code. If someone wants to use, for instance, pyyaml module, they can use pip to install the module in their src directory as such:

(delphix) ➜  my_plugin ls
artifact.json     plugin_config.yml schema.json       src
(delphix) ➜  my_plugin pip install pyyaml -t src/
DEPRECATION: Python 2.7 will reach the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 won't be maintained after that date. A future version of pip will drop support for Python 2.7.
Collecting pyyaml
  Downloading https://files.pythonhosted.org/packages/e3/e8/b3212641ee2718d556df0f23f78de8303f068fe29cdaa7a91018849582fe/PyYAML-5.1.2.tar.gz (265kB)
     |████████████████████████████████| 266kB 6.2MB/s
Building wheels for collected packages: pyyaml
  Building wheel for pyyaml (setup.py) ... done
  Stored in directory: /../Library/Caches/pip/wheels/d9/45/dd/65f0b38450c47cf7e5312883deb97d065e030c5cca0a365030
Successfully built pyyaml
Installing collected packages: pyyaml
Successfully installed pyyaml-5.1.2

Now, in the src directory, we have:

src/
   plugin_runner.py
   PyYAML-5.1.2.dist-info
   generated/
   yaml/

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