Add specific guidance on pull requests, especially in regard to tests

README.md

@@ -29,6 +29,15 @@ After you've finished, keep in mind that you're now overriding a default package
 
 ## Pull Requests
 
+Pull requests should:
+
+ 1. Start with the name of the package in square brackets, e.g. `[XML]`.
+ 2. Be as small and self-contained as is reasonable. *Smaller changes are quicker to review.*
+ 3. Include a new, or enhanced, [syntax test](http://www.sublimetext.com/docs/3/syntax.html) when changing a `.sublime-syntax` file.
+ 4. Have multiple syntax tests and a set of performance measurements (old vs. new) when making significant changes.
+
+### New Packages
+
 Pull requests for new packages won't be accepted at this stage, as new packages can cause issues for users who have a package with the same name installed via Package Control. There are some planned changes that will address this in the future.
 
 Complex plugins such as auto complete systems and linters should generally be in a stand-alone package, rather than included in the default set of packages.