Add PULL_REQUEST_TEMPLATEs

Problem

We want to make it as easy as possible for our contributors to do the
right thing.

Solution

Use Github's PR templates[1] to help them do this.

Result

Pull requests are more likely to be formatted correctly.

[1] https://help.github.com/articles/creating-a-pull-request-template-for-your-repository/

RB_ID=808946

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,15 @@
+Problem
+
+Explain the context and why you're making that change.  What is the
+problem you're trying to solve? In some cases there is not a problem
+and this can be thought of being the motivation for your change.
+
+Solution
+
+Describe the modifications you've done.
+
+Result
+
+What will change as a result of your pull request? Note that sometimes
+this section is unnecessary because it is self-explanatory based on
+the solution.

CONTRIBUTING.md

@@ -49,22 +49,26 @@ Comments should be formatted to a width no greater than 80 columns.
 Files should be exempt of trailing spaces.
 
 We adhere to a specific format for commit messages. Please write your commit
-messages along these guidelines:
+messages along these guidelines. Please keep the line width no greater than
+80 columns (You can use `fmt -n -p -w 80` to accomplish this).
 
     One line description of your change (less than 72 characters)
 
     Problem
 
-    Explain here the context, and why you're making that change.
-    What is the problem you're trying to solve?
+    Explain the context and why you're making that change.  What is the
+    problem you're trying to solve? In some cases there is not a problem
+    and this can be thought of being the motivation for your change.
 
     Solution
 
     Describe the modifications you've done.
 
     Result
 
-    After your change, what will change?
+    What will change as a result of your pull request? Note that sometimes
+    this section is unnecessary because it is self-explanatory based on
+    the solution.
 
 ## Code Review