GH-46215: [C++][Docs] Add README for Meson subprojects directory

Author: WillAyd

cpp/subprojects/README.md

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+  Licensed to the Apache Software Foundation (ASF) under one
+  or more contributor license agreements.  See the NOTICE file
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+  "License"); you may not use this file except in compliance
+  with the License.  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing,
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+  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+  KIND, either express or implied.  See the License for the
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+  under the License.
+-->
+
+# Meson Subprojects
+
+The contents of this directory are exclusively used by the Meson build configuration to
+manage project dependencies. The Meson documentation for this functionality can be found
+at https://mesonbuild.com/Subprojects.html
+
+To summarize, Arrow relies upon other projects to successfully compile and link. In
+the case that those dependencies cannot be found on the host system, Meson by convention
+forces users to place those dependencies in the `subprojects` directory at the root
+of the project.
+
+The easiest way to populate subprojects is to use Meson's
+[WrapDB system](https://mesonbuild.com/Wrapdb-projects.html). To illustrate how this
+works, let's take a look at the `googletest` library that Arrow depends upon for
+its test system. To create that as a subproject, a developer once ran:
+
+```bash
+meson wrap install gtest
+```
+
+From the project root directory. From that invocation, Meson creates the
+`subprojects/gtest.wrap` file which instructs the build system where it can
+get the source for gtest (if required), optionally alongside any "patch files"
+required to build gtest. If a project uses Meson natively, there is no need for
+patch files. However, if the project uses another build system (in the case of gtest
+, Bazel or CMake), then the patch files are user-created Meson configuration files
+that still expose the required build targets, without being a full rewrite of the
+native build generator. For an example of a user-created patch file for googletest,
+check out
+https://github.com/mesonbuild/wrapdb/tree/9e3862083a250680061aa46e8746499c419ad43c/subprojects/packagefiles/gtest
+
+If you depend upon a project that is not available in Meson's WrapDB system,
+you may still be able to have Meson auto-generate a wrapper for it. An example
+of this is the `subprojects/azure.wrap`, which looks like:
+
+```
+[wrap-file]
+source_url = https://github.com/Azure/azure-sdk-for-cpp/archive/azure-identity_1.9.0.tar.gz
+source_filename = azure-sdk-for-cpp-azure-identity_1.9.0.tar.gz
+source_hash = 97065bfc971ac8df450853ce805f820f52b59457bd7556510186a1569502e4a1
+directory = azure-sdk-for-cpp-azure-identity_1.9.0
+method = cmake
+```
+
+The `method = cmake` line is important here; it instructs Meson to inspect any
+CMakeLists.txt files from the downloaded source and auto generate Meson configuration
+files therefrom. The generated meson.build configuration(s) will be placed in
+`<build_directory>/subprojects/<subproject_name>` at project configuration time.
+
+In the default case, Meson will use the wrap file as a fallback. If a dependency
+can be satisfied by the system, then it will not use the wrap file to download
+any sources. However, you can toggle the behavior of the wrap system via the
+`--wrap-mode=` configuration option. `--wrap-mode=forcefallback` will always
+download and use the source defined in a wrap file, even if the depdendency could
+be satisfied by the system. By contrast, `--wrap-mode=nofallback` will require
+that the system satisfies dependencies. For more ways to handle wrap dependencies,
+see https://mesonbuild.com/Subprojects.html#commandline-options
+
+For more information on Meson's wrap system, see also
+https://mesonbuild.com/Wrap-dependency-system-manual.html
+
+In the majority of cases you will be using wrap files to describe subprojects, although
+it is not always required. You could alternatively place a copy of the third party
+project into the subprojects directory, if your preference is to completely vendor it.