More closely align the PR template with level commits

Changes include:

 - Writing out level branches in full
 - Adding a Purpose section
 - Adding a Walkthrough section
 - Specifying how tags should be indicated

The example in meta/contributing is also mentioned.

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,16 +1,20 @@
 <!-- Use "Level XX" as the title. Replace "XX" with the level number. -->
+<!-- A filled-out example can be found in the `meta/contributing` branch. -->
 
 Have you...
+- [ ] Read the CONTRIBUTING doc? (See the `meta/contributing` branch.)
 - [ ] Updated the `latest` file?
 - [ ] Added a hint to the `hints` directory?
 
 <!-- Next level code -->
 <details><summary>Next code</summary>
+
 <!-- Replace this line with the next level code. -->
 </details>
 
 <!-- False level codes -->
 <details><summary>False codes</summary>
+
 <!-- Replace this list with "NONE" if your level does not have any false codes. -->
 
 * falsecode1
@@ -19,11 +23,32 @@ Have you...
 
 <!-- Level branches -->
 <details><summary>Level branches</summary>
+<!--
+  Replace xxxxxxx with the level code.
+  Add more to the list below if your level has additional branches.
+  If you make use of tags, prefix it with `tag: `.
+-->
+
+* game/xxxxxxx/master
+</details>
+
+<!-- Purpose -->
+<details><summary>Purpose</summary>
+
+<!--
+  Talk about what major Git commands or concepts are being introduced or
+  are expected to be used together to solve the level.
+-->
+</details>
+
+<!-- Walkthrough -->
+<details><summary>Walkthrough</summary>
+<!-- Provide a step-by-step walkthrough here. -->
 
-* master
-<!-- Add more above if your level has additional branches -->
+1. Read the README. ...
+2. Check out `game/yyyyyyy/master`.
 </details>
 
 ## Comments
 
-<!-- Add optional notes below. Do not give a solution. -->
+<!-- Add optional notes below. These must be hint-free. -->