Bug 1628631 - Add a README.md file to devtools/docs to fix gitbook build r=Honza

Unless we plan to drop the gitbook version, we should avoid breaking the build.
GitBook relies on README.md to be available under the root folder. We can configure it to be a different filename, but it can't look in nested folders.

I just copied back the top content of index.rst. It's duplicating the content, but for now I don't see another option.

Differential Revision: https://phabricator.services.mozilla.com/D70326

--HG--
extra : moz-landing-system : lando

devtools/docs/README.md

@@ -0,0 +1,3 @@
+# Firefox Developer Tools: Contributor Docs
+
+This is a guide to working on the code for Firefox Developer Tools. If you're looking for help with using the tools, see the [user docs](https://developer.mozilla.org/en-US/docs/Tools). For other ways to get involved, check out our [community site](https://firefox-dev.tools).