Write contributing guide for bevy_lint

[BD103]

Since bevy_cli and bevy_lint are so separate, bevy_lint should have a separate contributing guide targeted specifically for it. It should cover the following topics at least:

1. Setup (installing toolchain, building and running the linter locally)
2. Finding lint ideas to work on, what is possible with a linter, and when a lint should not be implemented
3. Creating a lint (with a template file 📝)
4. Running and writing UI tests
5. Tips, tricks, and common patterns

• Finding specific type method calls
• Creating suggestions
• Useful queries
• Debugging, logging, and getting feedback from the compiler
• Figuring out what HIR node a lint can be silenced from
• Finding all types that implement a trait
• Getting the name of a type
• What do different LateLintPass methods check

6. Further resources (link to Clippy's documentation, rustc's developer guide, etc.)

I started this in #152, specifically in the bevy_lint/docs folder. Free free to start from there!