add CONTRIBUTING and explain how to run gh actions locally (#36)

Author: ktoso

CONTRIBUTING.md

@@ -0,0 +1,109 @@
+## Legal
+
+By submitting a pull request, you represent that you have the right to license
+your contribution to Apple and the community, and agree by submitting the patch
+that your contributions are licensed under the Apache 2.0 license (see
+`LICENSE.txt`).
+
+## How to submit a bug report
+
+Please ensure to specify the following:
+
+* Commit hash
+* Contextual information (e.g. what you were trying to achieve with swift-java)
+* Simplest possible steps to reproduce
+  * More complex the steps are, lower the priority will be.
+  * A pull request with failing test case is preferred, but it's just fine to paste the test case into the issue description.
+* Anything that might be relevant in your opinion, such as:
+  * Swift version or the output of `swift --version`
+  * OS version and the output of `uname -a`
+
+### Example
+
+```
+Commit hash: b17a8a9f0f814c01a56977680cb68d8a779c951f
+
+Context:
+While testing my application that uses with swift-openapi-generator, I noticed that ...
+
+Steps to reproduce:
+1. ...
+2. ...
+3. ...
+4. ...
+
+$ swift --version
+Swift version 4.0.2 (swift-4.0.2-RELEASE)
+Target: x86_64-unknown-linux-gnu
+
+Operating system: Ubuntu Linux 16.04 64-bit
+
+$ uname -a
+Linux beefy.machine 4.4.0-101-generic #124-Ubuntu SMP Fri Nov 10 18:29:59 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
+
+My system has IPv6 disabled.
+```
+
+## Writing a Patch
+
+A good patch is:
+
+1. Concise, and contains as few changes as needed to achieve the end result.
+2. Tested, ensuring that any tests provided failed before the patch and pass after it.
+3. Documented, adding API documentation as needed to cover new functions and properties.
+4. Accompanied by a great commit message, using our commit message template.
+
+### Run CI checks locally
+
+You can run the Github Actions workflows locally using
+[act](https://github.com/nektos/act). To run all the jobs that run on a pull
+request, use the following command:
+
+```
+% act pull_request
+```
+
+To run just a single job, use `workflow_call -j <job>`, and specify the inputs
+the job expects. For example, to run just shellcheck:
+
+```
+% act workflow_call -j soundness --input shell_check_enabled=true
+```
+
+To bind-mount the working directory to the container, rather than a copy, use
+`--bind`. For example, to run just the formatting, and have the results
+reflected in your working directory:
+
+```
+% act --bind workflow_call -j soundness --input format_check_enabled=true
+```
+
+If you'd like `act` to always run with certain flags, these can be be placed in
+an `.actrc` file either in the current working directory or your home
+directory, for example:
+
+```
+--container-architecture=linux/amd64
+--remote-name upstream
+--action-offline-mode
+```
+
+For frequent contributors, we recommend adding the script as a [git pre-push hook](https://git-scm.com/book/en/v2/Customizing-Git-Git-Hooks), which you can do via executing the following command in the project root directory: 
+
+```bash
+cat << EOF > .git/hooks/pre-push
+
+if [[ -f "scripts/soundness.sh" ]]; then
+  scripts/soundness.sh
+fi
+EOF
+```
+
+Which makes the script execute, and only allow the `git push` to complete if the check has passed.
+
+In the case of formatting issues, you can then `git add` the formatting changes, and attempt the push again. 
+
+## How to contribute your work
+
+Please open a pull request at https://github.com/swiftlang/swift-java. Make sure the CI passes, and then wait for code review.
+