doc: multiple README fixes
The IRC link is mentioned in CONTRIBUTING.md which is linked immediately above. We should encourage people to read that doc before hoping on the IRC channel.
Refs: https://github.com/nodejs/node/pull/16533#discussion_r147292940
Merge Working Groups with Contributing to Node.js.
Add Contributing to Node.js to the ToC and set it's heading level accordingly.
Normalize the ToC indentation with heading levels.
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc