Twitter OSS: Add ISSUE_TEMPLATE

Problem

Open source contributors don't have a standardized way of including information we
would like to be included when contributors open issues.

Solution

Create an ISSUE template for opening issues. The template is useful for customizing
and standardizing information we'd like to have contributors include when opening
an issue against a library.

Result

More help when creating issues in our Twitter OSS repositories.

RB_ID=849239

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,13 @@
+One line summary of the issue here.
+
+### Expected behavior
+
+As concisely as possible, describe the expected behavior.
+
+### Actual behavior
+
+As concisely as possible, describe the observed behavior.
+
+### Steps to reproduce the behavior
+
+Please list all relevant steps to reproduce the observed behavior.

CONTRIBUTING.md

@@ -62,6 +62,26 @@ locally).
 We generally follow [Effective Scala][es] and the [Scala Style Guide][ssg]. When
 in doubt, look around the codebase and see how it's done elsewhere.
 
+## Issues
+
+When creating an issue please try to ahere to the following format:
+
+    One line summary of the issue (less than 72 characters)
+
+    ### Expected behavior
+
+    As concisely as possible, describe the expected behavior.
+
+    ### Actual behavior
+
+    As concisely as possible, describe the observed behavior.
+
+    ### Steps to reproduce the behavior
+
+    List all relevant steps to reproduce the observed behavior.
+
+## Pull Requests
+
 Comments should be formatted to a width no greater than 80 columns.
 
 Files should be exempt of trailing spaces.