add CONTRIBUTING.md

Author: supersaiyansubtlety

CONTRIBUTING.md

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+# Contributing
+
+Thank you for your interest in contributing to Quilt Enigma Plugin!
+
+We recommend discussing your contribution with other members of the community - either directly in your pull request,
+or in our other community spaces. We're always happy to help if you need us!
+
+### Proposers
+
+Proposers should generate names that
+- follow [Quilt Mappings conventions](https://github.com/QuiltMC/quilt-mappings/blob/25w06a/CONVENTIONS.md)
+- consistently generate high-quality names
+
+Proposed names contribute to mapping stats, so it's better to leave things unmapped and easier to find
+than to propose names that need to be manually overriden.
+
+### Tests
+
+All proposers should be tested by an independent test class in `src/test/` in the same package as the proposer class.
+
+Most tests will require an obfuscated jar as input. *Do not* use other tests' jars or input classes;
+each test should be completely independent.
+
+To create the source for your obfuscated jar, create a directory under `src/testInputs/` named after your
+proposer class converted to `lowerCamelCase`, for example `MyNameProposer` -> `myNameProposer`.  
+
+Gradle will automatically create a source set from your directory and setup task dependencies to ensure your obfuscated
+jar is present for tests.
+
+Create input classes in your new test input source set,
+for example `src/testInputs/myNameProposer/java/com/example/MyProposerTestInput.java`
+
+Now on your test class in `src/test/`, implement `ConventionalNameProposerTest` and implement its
+`getTarget()` and `getTargetId()` methods so that they return the class and identifier of your proposer.
+
+Finally, in each test method, use `createAsserter()` to create a `ProposalAsserter` for your obfuscated jar.  
+Use the asserter to assert the presence/absence of mappings, and use its `remapper()` if you need to manually
+insert mappings in your test.
+
+### Descriptors, entries, and keeping track of obfuscated names
+When asserting mappings, you'll need to create obfuscated `Entry`s representing the obfuscated name of the
+class/member you're checking.  
+`TestUtil` has factory methods to streamline this process, and `CommonDescriptors` has reusable descriptors to pass to them.  
+
+You'll also likely need some of the obfuscated names of your test input classes. You can find them in the txt mapping file in
+`build/test-obf/` which is created by your test input obf task
+(ex: `myNameProposerTestObf` would create `myNameProposer-test-obf.txt`).
+
+However if you add or edit test input classes, obfuscated names may need to be updated.  
+Use variable names and comments to make it easier to track obfuscated names in your test class back to their test input sources.  
+For example, you might create a constant `private static final String MY_PROPOSER_TEST_INPUT_NAME = "a/a/a";` where the field name
+makes it clear that it represents the obfuscated name of `MyProposerTestInput`.