Docs: Update CONTRIBUTING.md

Update CONTRIBUTING.md to document the different components.

Notably, trivial should only be used for PRs that do not change the
code.

Author: jnewbery

CONTRIBUTING.md

@@ -53,7 +53,28 @@ about Git.
   - Create pull request
 
 The title of the pull request should be prefixed by the component or area that
-the pull request affects. Examples:
+the pull request affects. Valid areas as:
+
+  - *Consensus* for changes to consensus critical code
+  - *Docs* for changes to the documentation
+  - *Qt* for changes to bitcoin-qt
+  - *Mining* for changes to the mining code
+  - *Net* or *P2P* for changes to the peer-to-peer network code
+  - *RPC/REST/ZMQ* for changes to the RPC, REST or ZMQ APIs
+  - *Scripts and tools* for changes to the scripts and tools
+  - *Tests* for changes to the bitcoin unit tests or QA tests
+  - *Trivial* should **only** be used for PRs that do not change generated
+    executable code. Notably, refactors (change of function arguments and code
+    reorganization) and changes in behavior should **not** be marked as trivial.
+    Examples of trivial PRs are changes to:
+    - comments
+    - whitespace
+    - variable names
+    - logging and messages
+  - *Utils and libraries* for changes to the utils and libraries
+  - *Wallet* for changes to the wallet code
+
+Examples:
 
     Consensus: Add new opcode for BIP-XXXX OP_CHECKAWESOMESIG
     Net: Automatically create hidden service, listen on Tor