Add contribution guidelines (#608)

Author: AlexeySachkov

CONTRIBUTING.md

@@ -0,0 +1,83 @@
+# Contribution guidelines
+
+## If you have found a bug or would like to see a new feature
+
+Please reach us by creating a [new issue].
+
+Your bug report should include a proper description and steps to reproduce:
+- attach the LLVM BC or SPV file you are trying to translate and the command you
+  launch
+- any backtrace in case of crashes would be helpful
+- please describe what goes wrong or what is unexpected during translation
+
+For feature requests, please describe the feature you would like to see
+implemented in the translator.
+
+[new issue]: https://github.com/KhronosGroup/SPIRV-LLVM-Translator/issues/new
+
+## If you would like to contribute your change
+
+Please open a [pull request]. If you are not sure whether your changes are
+correct, you can either mark it as [draft] or create an issue to discuss the
+problem and possible ways to fix it prior to publishing a PR.
+
+It is okay to have several commits in the PR, but each of them should be
+buildable and tests should pass. Maintainers can squash several commits
+into a single one for you during merge, but if you would like to see several
+commits in the git history, please let us know in PR description/comments so
+maintainers will rebase your PR instead of squashing it.
+
+Each functional change (new feature or bug fix) must be supplied with
+corresponding tests. See [#testing-guidelines] for more information about
+testing. NFC (non-functional change) PRs can be accepted without new tests.
+
+Code changes should follow coding standards, which are inherited from [LLVM
+Coding Standards]. Compliance of your code is checked automatically using
+Travis CI. See [clang-format] and [clang-tidy] configs for more details about
+coding standards.
+
+### Conditions to merge a PR
+
+In order to get your PR merged, the following conditions must be met:
+- If you are a first-time contributor, you have to sign the
+  [Contributor License Agreement]. Corresponding link and instructions will be
+  automatically posted into your PR.
+- [Travis CI testing] jobs must pass on your PR: this includes functional
+  testing and checking for complying with coding standards.
+- You need to get approval from at least one contributor with merge rights.
+
+As a contributor, you should expect that even an approved PR might still be left
+open for a few days: this is needed, because the translator is being developed
+by different vendors and individuals and we need to ensure that each interested
+party is able to react to new changes and provide feedback.
+
+Information below is a guideline for repo maintainers and can be used by
+contributors to get some expectations about how long a PR has to be open before
+it can be merged:
+- For any significant change/redesign, the PR must be open for at least 5
+  working days, so everyone interested can step in to provide feedback, discuss
+  direction and help to find bugs.
+  - Ideally, there should be approvals from different vendors/individuals to get
+    it merged, particularly for larger changes.
+- For regular changes/bug fixes, the PR must be open for at least 2-3 working
+  days, so everyone interested can step in for review and provide feedback.
+  - If the change is vendor-specific (bug fix in vendor extension implementation
+    or new vendor-specific extension support), then it is okay to merge PR
+    sooner.
+  - If the change affects or might affect several interested parties, the PR
+    must be left open for 2-3 working days and it would be good to see feedback
+    from different vendors/inviduals before merging.
+- Tiny NFC changes or trivial build fixes (due to LLVM API changes) can be
+  submitted as soon as testing is finished and PR approved - no need to wait for
+  too long.
+- In general, just use common sense to wait long enough to get feedback from
+  everyone who might be interested in the PR and don't hesitate to explicitly
+  mention individuals who might be interested in reviewing the PR.
+
+[pull request]: https://github.com/KhronosGroup/SPIRV-LLVM-Translator/pulls
+[draft]: https://docs.github.com/en/github/collaborating-with-issues-and-pull-requests/about-pull-requests#draft-pull-requests
+[LLVM Coding Standards]: https://llvm.org/docs/CodingStandards.html
+[clang-format]: [.clang-format]
+[clang-tidy]: [.clang-tidy]
+[Contributor License Agreement]: https://cla-assistant.io/KhronosGroup/SPIRV-LLVM-Translator
+[Travis CI testing]: https://travis-ci.org/KhronosGroup/SPIRV-LLVM-Translator