Add Contributing guide (#20)

CONTRIBUTING.md

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+# Contribution Guide
+
+First of all, thank you for contributing! :3
+
+To ensure your contribution is robust, please follow the below guide!
+
+## Style Guide
+
+- This project has a very minimal .editorconfig. Make sure your editor supports this!
+  If you are using VSCode, it should automatically recommend you the extension; If not,
+  please install the Editorconfig extension
+- Try to follow the formatting in the rest of the project and stay consistent
+- Follow the file naming convention. File names should usually be camelCase, unless they export a Class
+  or React Component, in which case they should be PascalCase
+
+
+## Contributing a Plugin
+
+Because plugins modify code directly, incompatibilities are a problem.
+
+Thus, 3rd party plugins are not supported, instead all plugins are part of Vencord itself.
+This way we can ensure compatibility and high quality patches.
+
+Follow the below guide to make your first plugin!
+
+
+### Finding the right module to patch
+
+If the thing you want to patch is an action performed when interacting with a part of the UI, use React DevTools.
+They come preinstalled and can be found as the "Components" tab in DevTools.
+Use the Selector (top left) to select the UI Element. Now you can see all callbacks, props or jump to the source
+directly.
+
+If it is anything else, or you're too lazy to use React DevTools, hit `CTRL + Shift + F` while in DevTools and
+enter a search term, for example "getUser" to search all source files.
+Look at the results until you find something promising. Set a breakpoint and trigger the execution of that part of Code to inspect arguments, locals, etc...
+
+### Writing a robust patch
+
+##### "find"
+
+First you need to find a good `find` value. This should be a string that is unique to your module.
+If you want to patch the `getUser` function, usually a good first try is `getUser:` or `function getUser()`,
+depending on how the module is structured. Again, make sure this string is unique to your module and is not
+found in any other module. To verify this, search for it in all bundles (CTRL + Shift + F)
+
+##### "match"
+
+This is the regex that will operate on the module found with "find". Just like in find, you should make sure
+this only matches exactly the part you want to patch and no other parts in the file.
+
+The easiest way to write and test your regex is the following:
+- Get the ID of the module you want to patch. To do this, go to it in the sources tab and scroll up until you
+  see something like `447887: (e,t,n)=>{` (Obviously the number will differ).
+- Now paste the following into the console: `Vencord.Webpack.wreq.m[447887].toString()` (Changing the number to your ID)
+- Now either test regexes on this string in the console or use a tool like https://regex101.com
+
+Also pay attention to the following:
+- Never hardcode variable or parameter names or any other minified names. They will change in the future. The only Exception to this rule
+  are the react props parameter which seems to always be `e`, but even then only rely on this if it is necessary.
+  Instead, use one of the following approaches where applicable:
+  - Match 1 or 2 of any character: `.{1,2}`, for example to match the variable name in `var a=b`, `var (.{1,2})=`
+  - Match any but a guaranteed terminating character: `[^;]+`, for example to match the entire assigned value in `var a=b||c||func();`,
+    `var .{1,2}=([^;]+);`
+  - If you don't care about that part, just match a bunch of chars: `.{0,50}`, for example to extract the variable "b" in `createElement("div",{a:"foo",c:"bar"},b)`, `createElement\("div".{0,30},(.{1,2})\),`. Note the `.{0,30}`, this is essentially the same as `.+`, but safer as you can't end up accidently eating thousands of characters
+- Additionally, as you might have noticed, all of the appove approaches use regex groups (`(...)`) to capture the variable name. You can then use those groups in your replacement to access those variables dynamically
+
+#### "replace"
+
+This is the replacement for the match. This is the second argument to [String.replace](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/replace), so refer to those docs for info.
+
+Never hardcode minified variable or parameter names here. Instead, use capture groups in your regex to capture the variable names
+and use those in your replacement
+
+Make sure your replacement does not introduce any whitespace. While this might seem weird, random whitespace may mess up other patches.
+This includes spaces, tabs and especially newlines
+
+___
+
+And that's it! Now open a Pull Request with your Plugin