* Update README.md
Mention Discourse community earlier, add missing period
* Update CONTRIBUTING.md
i.e. is used for equivalence or clarification while e.g. is for
examples. For instance, we shouldn't say that IMAP is equivalent or an
explanation to all open-source software used to access email (e.g.
there's POP3, open-source clients to access when there isn't IMAP such
as Tutanota, etc.).
We also shouldn't call IMAP open-source software since it's a protocol.
* Various typo and grammar corrections.
* Use standard single and double quotes.
* Opt for American English, like the rest of the text.
* Read contributing guide before making PR and fix broken links there too ;-)
* Fix broken links and favour HTTPS where possible.
