Create issue template

This template will help users potentially troubleshoot parity issues
themselves and will provide us with more information about issues that
we frequently request.

cc: #137

.github/issue_template.md

@@ -0,0 +1,16 @@
+Help us track down and resolve parity problems faster with this template.
+
+## What command did you execute?
+
+## What did you expect to happen?
+
+## What actually happened?
+
+## Some information about your installation
+
+- What's your operating system?
+- What's the output of `which development`, `which staging`, `which production`?
+  Parity has had multiple installation channels, and it's not uncommon for an
+  old version to be somewhere else in your path.
+- If installed via Homebrew, what does `brew list parity` output?
+- If installed via Rubygems, what's the gem version?

README.md

@@ -144,6 +144,11 @@ else
 end
 ```
 
+Issues
+------
+Please fill out our [issues template](.github/issue_template.md) if you are
+having problems.
+
 Contributing
 ------------