Restructuring and Updating CONTRIBUTING.md (#312)

Alines with the terms from the README.md (e.g. CSL-JSON Schema or 
add csl-categories.rnc).

Also makes explicit a section about issues (with the text which 
was already there) and moved this together with PR on the top. 

The rules for versioning are much less frequent and therefore 
following, but also renamed from their previous title.

Author: zuphilip

CONTRIBUTING.md

@@ -1,31 +1,33 @@
 # How to Contribute
 
-## Schema Changes
+## Pull Requests
+
+We encourage pull requests. Please use the following guidelines:
+
+- in general, follow [these recommendations](https://www.freecodecamp.org/news/writing-good-commit-messages-a-practical-guide/) on writing good commit messages
+- make sure your PRs are focused, and link where possible to existing issues
+- follow  the instructions on the PR template
+
+## Issues
+
+Regardless of the change, however, the current status of CSL requires us to be fairly conservative. For issue reports, please follow the template instructions, and include all specified information. It's important for us to be able to quickly understand what you are requesting, how broad the need is, and what implementation options there are.
+
+## Versioning
 
-### RELAX NG Style Language
+### Changes to the CSL Schema
 
 At this point, changes to the CSL RELAX NG schema consist of the following:
 
 <dl>
-  <dt>Changes to csl-terms.rnc, csl-types.rnc, or csl-variables.rnc</dt>
+  <dt>Changes to csl-categories.rnc, csl-terms.rnc, csl-types.rnc, or csl-variables.rnc</dt>
   <dd>Adding new variables, item types, locators, or terms; these can usually be added to minor x.x.x version changes.</dd>
   <dt>Changes to csl.rnc</dt>
   <dd>These are often more significant and may have an impact on compatability, and so are typically reserved for major x.x version changes.</dd>
 </dl>
 
-Regardless of the change, however, the current status of CSL requires us to be fairly conservative. For issue reports, please follow the template instructions, and include all specified information. It's important for us to be able to quickly understand what you are requesting, how broad the need is, and what implementation options there are.
-
-### JSON Data Schemas
-
-The data schemas are intended to mirror the rnc files, and so follow similar conventions. Ideally when we add variables, we add to both at the same time.
-
-## Pull Requests
+### Changes to the CSL-JSON Schema
 
-We encourage pull requests. Please use the following guidelines:
-
-- in general, follow [these recommendations](https://www.freecodecamp.org/news/writing-good-commit-messages-a-practical-guide/) on writing good commit messages
-- make sure your PRs are focused, and link where possible to existing issues
-- follow  the instructions on the PR template
+The data schemas are intended to mirror the `rnc` files, and so follow similar conventions. Ideally when we add variables, we add to both at the same time.
 
 ## Pre-Commit Hooks