docs: add CONTRIBUTING.md (anime-dl#719)

CONTRIBUTING.md

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+# Contributing to Support
+
+Thank you for taking the time to contribute. Please read the [CODE of CONDUCT](CODE_OF_CONDUCT.md).
+As a contributor, here are the guidelines we would like you to follow:
+
+- [Commit Message Guidelines](#commit)
+
+---
+
+## <a name="commit"></a> Commit Message Guidelines 😎
+
+In order to make git commit messages **easier to read** and faster to reason about, we follow some guidelines on most commits to keep the **format predictable**. Check [Conventional Commits specification](https://conventionalcommits.org) for more information about our guidelines.
+
+**Examples**:
+
+```
+docs(changelog): update changelog to beta.5
+docs: add API documentation to the bot
+test(server): add cache tests to the movie resource
+fix(web): add validation to phone input field
+fix(web): remove avatar image from being required in form
+fix(release): need to depend on latest rxjs and zone.js
+```
+
+### Type
+
+Must be one of the following:
+
+- **build**: Changes that affect the build system or external dependencies (example scopes: gulp, broccoli, npm)
+- **ci**: Changes to our CI configuration files and scripts (example scopes: Circle, BrowserStack, SauceLabs)
+- **docs**: Documentation only changes
+- **feat**: A new feature
+- **fix**: A bug fix
+- **perf**: A code change that improves performance
+- **refactor**: A code change that neither fixes a bug nor adds a feature
+- **style**: Changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc)
+- **test**: Adding missing tests or correcting existing tests