Add contributing guide

CONTRIBUTING.md

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+# Contributing to wayvnc
+
+## Commit Messages
+
+Please, try to write good commit messages. Do your best to follow these 7 rules,
+borrowed from [Chris Beams][https://chris.beams.io/posts/git-commit/]:
+
+ 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
+
+If you wish to know why we follow these rules, please read Chris Beams' blog
+entry, linked above.
+
+## Style
+
+This project follows the the
+[Linux kernel's style guide][https://www.kernel.org/doc/html/latest/process/coding-style.html#codingstyle]
+as far as coding style is concererned, with the following exceptions:
+
+ * When declaring pointer variables, the asterisk (`*`) is placed on the left
+   with the type rather than the variable name. Declaring multiple variables in
+   the same line is not allowed.
+ * Wrapped argument lists should not be aligned. Use two tabs instead. There is
+   a lot of code that uses aligned argument lists in the project, but I have
+   come to the conclusion that these alignments are not very nice to maintain.
+
+## No Brown M&Ms
+
+All pull requests must contain the following sentence in the description:
+I have read and understood CONTRIBUTING.md and its associated documents.