Add contributing guidelines

CONTRIBUTING.md

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+Contributions to mode-icons are always welcomed and appreciated. To
+help accept your contributions into mode-icons please consider the
+following guidelines before sending a pull request.
+
+For code:
+
+- Use only spaces for indentation, don't use tabs.
+- Try not to exceed a length of 80 characters on a single line.
+  Definitely don't exceed a length of 100 characters.
+- Update the CHANGELOG.md file when you add, change, fix or delete
+  something users of mode-icons might notice.
+
+For commits:
+
+- Please keep your commits focused on a single topic. Don't fix a bug
+  and include a new icon in a single commit, for example.
+
+Feel free to discuss these guidelines with me, but I won't promise
+I'll change them.
+
+If you find any code that doesn't adhere to these guidelines, please
+feel free to open a pull request with changes that fix it.