Initial version of contributing guide. (rust-lang#506)

* Add initial version of contributing guide.
* Highlight the preferential usage of the Enzyme compiler explorer to explore bugs and issues.

CONTRIBUTING.md

@@ -0,0 +1,51 @@
+# Contributing Guide
+
+Thank you for your interest in contributing to Enzyme! There are multiple ways to contribute, and we appreciate all contributions. In case you have questions, you can either use the [Discourse](https://discourse.llvm.org/c/projects-that-want-to-become-official-llvm-projects/enzyme/45) or the #enzyme channel on [Discord](https://discord.gg/xS7Z362).
+
+
+## Ways to Contribute
+
+### Bug Reports and Feature Requests
+
+If you are working with Enzyme, and run into a bug, or require additional features, we definitely want to know about it. Please let us know by either creating a [GitHub Issues](https://github.com/EnzymeAD/Enzyme/issues), or starting a thread in Enzyme's [LLVM Discourse Category](https://discourse.llvm.org/c/projects-that-want-to-become-official-llvm-projects/enzyme/45).
+
+### Bug Fixes
+
+If you are interested in contributing code to Enzyme, issues labeled with the [good-first-issue](https://github.com/EnzymeAD/Enzyme/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22) keyword in the [GitHub Issues](https://github.com/EnzymeAD/Enzyme/issues) are a good way to get familiar with the code base. If you are interested in fixing a bug please comment on it to let people know you are working on it.
+
+> The easiest way to explore code in Enzyme is by using our [Compiler Explorer](https://enzyme.mit.edu/explorer) instance.
+
+If you are unable to reproduce the bug in Compiler Explorer, try to reproduce and fix the bug with upstream LLVM. Start by building LLVM from source as described in [Installation](https://enzyme.mit.edu/Installation/) and use the built binaries to reproduce the failure described in the bug.
+
+### Language Integrations
+
+In case you are interested in integrating Enzyme with a programming language which compiles to the LLVM IR, we encourage you to take a look at the [Language Frontend Guide](), and post on the [Discourse Category](https://discourse.llvm.org/c/projects-that-want-to-become-official-llvm-projects/enzyme/45) to let people know that you are working on it.
+
+## How to Submit a Pull Request
+
+Once you have a pull request ready, it is time to submit it. The pull request should:
+
+- Include a small unit test
+- Conform to the [LLVM Coding Standards](https://llvm.org/docs/CodingStandards.html). You can use the [clang-format-diff.py](https://reviews.llvm.org/source/llvm-github/browse/main/clang/tools/clang-format/clang-format-diff.py) or [git-clang-format](https://reviews.llvm.org/source/llvm-github/browse/main/clang/tools/clang-format/git-clang-format) tools to automatically format your patch properly.
+- Not contain any unrelated changes
+- Be an isolated change. Independent changes should be submitted as separate pull requests as this makes reviewing easier.
+
+Before sending a pull request for review, please also try to ensure it is formatted properly. We use clang-format for this, which has git integration through the git-clang-format script. On some systems, it may already be installed (or be installable via your package manager). If so, you can simply run it – the following command will format only the code changed in the most recent commit:
+
+```bash
+git clang-format HEAD~1
+```
+
+Note that this modifies the files, but doesn't commit them - you'll likely want to run
+
+```bash
+git commit --amend -a
+```
+
+in order to update the latest commit with all pending changes.
+
+> If you don't already have `clang-format` or `git-clang-format` installed on your system, the `clang-format` binary will be built alongside clang, and the git integration can be run form `clang/tools/clang-format/git-clang-format`.
+
+## Code of Conduct
+
+As an LLVM Incubator Project we expect all participants to abide by the [LLVM Community Code of Conduct](https://llvm.org/docs/CodeOfConduct.html).