Document how to update SQLite source dependencies.

README.rst

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+cpython-source-deps
+===================
+
+Source for packages that the CPython build process depends on.
+
+It is currently expected that this will only be useful on Windows,
+and in any case you should never need to clone this repository
+unless you are updating its contents.
+
+.. contents::
+
+Updating Source Dependencies
+----------------------------
+
+The procedure for updating the different source dependencies are similar.  Below
+is an example for updating SQLite::
+
+
+1. Fork and clone this repository.
+
+2. Checkout a new branch off the ``sqlite`` branch. Assuming this repo is set
+   as your ``upstream``::
+
+      git checkout -b <branch-name>-sqlite upstream/sqlite
+
+3. Download SQLite source from `sqlite.org <https://www.sqlite.org>`_.
+
+4. Unzip it to the branch checked out in step 2.
+
+5. Commit and push the changes.
+
+6. Create the PR, with ``sqlite`` as the base branch.
+
+Once the PR has been merged, tag the commit as ``sqlite-<full version to be used in CPython's PCbuild/get_externals.bat>``.
+
+For updating ``sqlite``, ``bzip2``, ``xz``, and ``zlib``, follow the above
+instructions.
+
+For ``tcl`` and ``tk``, after step 5, `cpython-bin-deps
+<https://github.com/python/cpython-bin-deps/>`_
+needs to be updated.
+
+For ``openssl``, step 5 will be more complicated :)
+
+
+Download links
+--------------
+
+- ``sqlite``: https://www.sqlite.org/download.html
+- ``bzip2``: http://www.bzip.org/downloads.html
+- ``xz``: https://tukaani.org/xz/
+- ``zlib``: https://zlib.net/
+- ``tcl``/``tk``: https://tcl.tk/software/tcltk/download.html
+- ``openssl``: https://www.openssl.org/source/
+
+
+Tagging the commit
+------------------
+
+Using the ``sqlite`` branch as an example::
+
+   git checkout -b sqlite-tag upstream/sqlite
+   git tag sqlite-3.21.0.0  # replace 3.21.0.0 with the correct version.
+   git push --tags upstream
+