This allows one to specify the options passed to `Apply.clone_with_new_inputs`.

- 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.