- Add "Contribution Expectations" and "Issues and Discussions" sections
- Remove section on migrating PRs from Theano

* Also reverses test expectation for `local_expm1` rewrite, as previously missed case is now detected

Closes #1255

Although the information is burried in another part of the
documentation, I could not find any instructions in the the developer
starting guide. Since this questions has been asked multiple times in the
issue and PR tracker on the repo I added short instructions
regarding documentation contributions.

The current installation (and update) instructions are out of date, the
update instructions are not giving any useful information, and the
developer installation instructions are redudundant with the developer
starting guide.

I thus removed most of the instructions and replaced them with the instructions that
are currently in the README.