Add note on editing docs in vim

@kaapstorm 
Note that this PR is into the sphinx_docs branch (not master), to get into PR https://github.com/dimagi/commcare-hq/pull/4785/
lol.  It is if you're used to the individual components, actually.  It's a sequence of commands.  "**yy** ank line, **p** aste, **V** isually select line, **r** eplace with `=`"

Capital letters or doubles do the action to the current line (you could do the same thing with `YpVr=` if you felt so inclined).

The analagous command in another editor would be something like `click click click Ctrl+c End Enter Ctrl+v click click click Ctrl+r ="  (assuming there's a "replace" command), or more likely, "End Enter =(hold) Backspace(as needed)"