Add description on the vendoring process we use (#22204)

We need to vendor in cgroupspy library in order to make Airflow
compatible with Python 3.10 (see #22050) so this is the right time
to make our vendoring process more organized.

I based in parts on the readme described by `bleach` package.

airflow/_vendor/README.md

@@ -0,0 +1,37 @@
+# Vendor package
+
+## What vendored packages are for
+
+The `airflow._vendor` package is foreseen for vendoring in packages, that we have to modify ourselves
+because authors of the packages do not have time to modify them themselves. This is often temporary
+and once the packages implement fixes that we need, and then we remove the packages from
+the `_vendor` package.
+
+All Vendored libraries must follow these rules:
+
+1. Vendored libraries must be pure Python--no compiling (so that we do not have to release multi-platform airflow packages on PyPI).
+2. Source code for the libary is included in this directory.
+3. License must be included in this repo and in the [LICENSE](../../LICENSE) file and in the
+   [licenses](../../licenses) folder.
+4. Requirements of the library become requirements of airflow core.
+5. Version of the library should be included in the [vendor.md](vendor.md) file.
+6. No modifications to the library may be made in the initial commit.
+7. Apply the fixes necessary to use the vendored library as separate commits - each package separately,
+   so that they can be cherry-picked later if we upgrade the vendored package. Changes to airflow code to
+   use the vendored packages should be applied as separate commits/PRs.
+8. The `_vendor` packages should be excluded from any refactorings, static checks and automated fixes.
+
+## Adding and upgrading a vendored package
+
+Way to vendor a library or update a version:
+
+1. Update ``vendor.txt`` with the library, version, and SHA256 (`pypi` provides hashes as of recently)
+2. Remove all old files and directories of the old version.
+3. Replace them with new files (only replace relevant python packages:move LICENSE )
+   * move licence files to [licenses](../../licenses) folder
+   * remove README and any other supporting files (they can be found in PyPI)
+   * make sure to add requirements from setup.py to airflow's setup.py with appropriate comment stating
+     why the requirements are added and when they should be removed
+4. If you replace previous version, re-apply historical fixes from the "package" folder by
+   cherry-picking them.
+