docs: README: specify how to contribute instead of pointing at another file

This repository is partially included in another repository: poky.

However its README isn't making it, so documentation/README pointing at
the README at the root of the git repository would lead the contributor
nowhere.

Instead, let's include the appropriate information directly in
documentation/README which does make it to the poky git repo.

(From yocto-docs rev: 1b93a4122624b554243842822e7df209cc884b27)

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
(cherry picked from commit 0298318cea2947e65754eab97255164e64a862de)
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/README

@@ -406,5 +406,22 @@ both the Yocto Project and BitBake manuals:
 Submitting documentation changes
 ================================
 
-Please see the top level README file in this repository for details of where
-to send patches.
+Please refer to our contributor guide here: https://docs.yoctoproject.org/contributor-guide/
+for full details on how to submit changes.
+
+As a quick guide, patches should be sent to docs@lists.yoctoproject.org
+The git command to do that would be:
+
+     git send-email -M -1 --to docs@lists.yoctoproject.org
+
+The 'To' header can be set as default for this repository:
+
+     git config sendemail.to docs@lists.yoctoproject.org
+
+Now you can just do 'git send-email origin/master..' to send all local patches.
+
+Read the other sections in this document and documentation/standards.md for
+rules to follow when contributing to the documentation.
+
+Git repository: https://git.yoctoproject.org/yocto-docs
+Mailing list: docs@lists.yoctoproject.org