Create CONTRIBUTING.md (#15)

CONTRIBUTING.md

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+# Contributing guide
+
+## Commit guidelines
+
+### Subject line content
+
+The subject line of a commit should explain what files or topic in the project
+is being modified as well as the overall change that is being made.
+
+For example, if the overall documentation format is being changed to meet a new
+standard, a good subject line would be as follows:
+
+```
+documentation: rework to fit standard X
+```
+
+Another example, if a specific documentation file is being changed to fix
+spelling errors, a good subject line would be as follows:
+
+```
+documentation/some-file.md: fix spelling errors
+```
+
+### Subject line format
+
+The subject line should be no longer than 72 characters. Past this it will wrap
+when reading via a standard sized terminal. If the changes cannot be summarized
+in that length then the commit is likely better split into multiple commits.
+
+The `topic: summary of what changed` format is preferred but will not be
+enforced. As long as it includes the topic and a summary of what changed it is
+acceptable.
+
+### Body content
+
+The body of the commit should explain the why the commit is needed or wanted.
+
+The body may also give more detail on what is being changed or how it is being
+changed.
+
+With simple and obvious commits this is not always necessary and the body of the
+commit may be omitted.
+
+### Body format
+
+Body text should usually not go past 72 characters per line. This is not a hard
+rule and can be broken where appropriate. For example, if error text is included
+in the commit body and is longer than 72 characters, it does not need to be
+broken into shorter lines.