docs: copy editor setup instructions from wiki to docs/editors.md (#4504)

**What type of PR is this?**

> Documentation

**What does this PR do? Why is it needed?**

And link from README.rst.

GitHub wikis seem to be downranked pretty hard by Google, so these
instructions haven't been very visible. Linking from the README should
help.

**Which issues(s) does this PR fix?**

n/a

**Other notes for review**

For some reason, the vim instructions were in docs/go/editors/vim.md
instead of in the wiki. I've squashed them into a new section here.