Adding a contribution guide

The GitLab interface also supports hosting a contribution guide.
This guide is fairly basic, but outlines three main points
1. Talk with Chris Subich before merging changes
2. How to submit a merge request via GitLab
3. Version number etiquette

CONTRIBUTING.md

+# Contributing to SPINS
+
+SPINS is maintained by Chris Subich (@csubich).
+If you have made developments to SPINS that you would like to include in the main repository,
+first contact Chris to discuss the matter with him.
+
+## Merging Changes
+
+The uWaterloo GitLab system (<https://git.uwaterloo.ca>, the current host of SPINS) supports merge requests.
+Once you have settled on changes that you would like to merge, use the following steps to prepare the merge request.
+1. Create a fork of the main SPINS repository if you don't already have one
+2. Apply your changes to the clean fork. Make sure that you follow good commit standards and
+  1. Commit incremental changes when possible
+  2. Leave informative commit messages
+  3. Update the version number as appropraite (see section on Version Numbers)
+3. Once you've commit all of your changes to your fork, go to the main page for SPINS (<https://git.uwaterloo.ca/SPINS/SPINaS_main>)
+  1. Select the *Merge Resquests* tab
+  2. Choose *New Merge Request*
+  3. Select the appropriate fork/branch as the source
+  4. Leave an informative merge message that summarizes the changes being merged.
+
+
+## Version Numbers
+
+Version numbers are tracked in the file VERSION (which are then automatically passed to
+the executables at compile-time).
+This means that changing the version number is as simple as updating the VERSION file.
+
+When pushing changes to the main SPINS repository, you should increment the version numbers accordingly.
+
+StackOverflow has a nice summary of when to change which version number
+(https://stackoverflow.com/questions/3826580/what-rules-does-software-version-numbering-follow)
+- Major version numbers change whenever there is some significant change  being introduced. For example, a large or potentially backward-incompatible change to a software package.
+- Minor version numbers change when a new, minor feature is introduced or when a set of smaller features is rolled out.
+- Patch numbers change when a new build of the software is released to customers. This is normally for small bug-fixes or the like.