Add commit message guidelines to contributing

contributing.md

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+#### Write a good commit message
+Here are a few rules to keep in mind while writing a commit message
+
+   1. Separate subject from body with a blank line
+   2. Limit the subject line to 50 characters
+   3. Capitalize the subject line
+   4. Do not end the subject line with a period
+   5. Use the imperative mood in the subject line
+   6. Wrap the body at 72 characters
+   7. Use the body to explain what and why vs. how
+
+ A good commit message looks something like this
+```
+  Summarize changes in around 50 characters or less
+
+ More detailed explanatory text, if necessary. Wrap it to about 72
+ characters or so. In some contexts, the first line is treated as the
+ subject of the commit and the rest of the text as the body. The
+ blank line separating the summary from the body is critical (unless
+ you omit the body entirely); various tools like `log`, `shortlog`
+ and `rebase` can get confused if you run the two together.
+
+ Explain the problem that this commit is solving. Focus on why you
+ are making this change as opposed to how (the code explains that).
+ Are there side effects or other unintuitive consequences of this
+ change? Here's the place to explain them.
+
+ Further paragraphs come after blank lines.
+
+  - Bullet points are okay, too
+
+  - Typically a hyphen or asterisk is used for the bullet, preceded
+    by a single space, with blank lines in between, but conventions
+    vary here
+
+ If you use an issue tracker, put references to them at the bottom,
+ like this:
+
+ Resolves: #123
+ See also: #456, #789
+
+```
+This is taken from [chris beams git commit](https://chris.beams.io/posts/git-commit/).
+You may want to read this for a more detailed explanation (and links to other posts on
+how to write a good commit message).
+This content is licensed under [CC-BY-SA](https://creativecommons.org/licenses/by-sa/4.0/).
+