Add contribution guidelines

CONTRIBUTING.md

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+# Contributing
+
+Contributions are very welcome! Here are some guidelines on how the project is designed.
+
+### CodeStyle
+
+- Adhere to PEP8 as much as possible.
+
+- Line lengths should be under 120 characters, use list comprehensions over map/filter, don't leave trailing whitespace.
+
+- More complex pieces of code should be commented for future reference.
+
+### Structure
+
+There are a few self-imposed rules on the project structure, to keep the project as tidy as possible.
+- All modules should go into the `modules/` directory.
+- Any database accesses should be done in `modules/sql/` - no instances of SESSION should be imported anywhere else.
+- Make sure your database sessions are properly scoped! Always close them properly.
+- When creating a new module, there should be as few changes to other files as possible required to incorporate it.
+Removing the module file should result in a bot which is still in perfect working condition.
+- If a module is dependent on multiple other files, which might not be loaded, then create a list of at module
+load time, in `__main__`, by looking at attributes. This is how migration, /help, /stats, /info, and many other things
+are based off of. It allows the bot to work fine with the LOAD and NO_LOAD configurations.
+- Keep in mind that some things might clash; eg a regex handler could clash with a command handler - in this case, you 
+should put them in different dispatcher groups.
+
+Might seem complicated, but it'll make sense when you get into it. Feel free to ask me for a hand/advice!