add contributing

Author: spences10

CONTRIBUTING.md

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+# Feel free to contribute.
+
+Please bear in mind this project is barely maintained.
+
+If making a PR I need a good explanation of what the intended change
+is for.
+
+If it's not clear (to me) what the change is doing then I have no time
+to review and test and will more than likely close the PR, see
+previous paragraph.
+
+Reach out via issue explaining what you propose before doing any work.
+
+If you are looking for language completion see [Excel Live Server]
+(untested by me).
+
+<!-- Links -->
+
+[excel live server]:
+  https://marketplace.visualstudio.com/items?itemName=local-smart.excel-live-server