vshard: contribution short guide

[TarantoolBot]

Product: modules
Since:
Audience/target:
Root document: https://www.tarantool.io/en/doc/latest/contributing/contributing/#how-to-contribute-to-modules
SME: @ Gerold103

Details

The guide copypaste from README:

---

In order to contribute you might want to avoid installation into regular paths.
You need to fetch the source code to patch it in a local folder.

• git clone <this repo or your fork>;
• git submodule update --init --recursive;
• VShard requires Tarantool being in PATH. So either you install one into the
  system or you fetch Tarantool's main repository source code, build it, and
  add to PATH manually these paths: <path to tarantool build>/src and
  <path to tarantool build>/extra.

Now vshard should be functional. You can try it in example folder, see its
Makefile.

Your patch should pass all the existing tests (unless it is necessary to change
them) and have its own test usually. To run the tests this should work:

• cd test;
• python test-run.py or ./test-run.py;

[veod32]

Discussed and decided that it's not a good idea to duplicate the same content in two different places. In this case, the content becomes out of synch eventually. It's better to keep this guide in the repository readme.