Add a contributing guide

CONTRIBUTING.md

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+# Contributing to the Parameterized Trigger Plugin
+
+## Beginner Topics
+
+Look for contribution areas in the
+[issues in our issues tracker](https://issues.jenkins.io/issues/?jql=resolution%20is%20EMPTY%20and%20component%3D15592).
+
+## Background
+
+Plugin source code is hosted on https://github.com/jenkinsci/parameterized-trigger-plugin[GitHub].
+New feature proposals and bug fix proposals should be submitted as https://help.github.com/articles/creating-a-pull-request[GitHub pull requests].
+Your pull request will be evaluated by the https://ci.jenkins.io/job/Plugins/job/parameterized-trigger-plugin/[Jenkins job].
+
+Before submitting your change, please assure that you've added tests to verify your change.
+Tests help us assure that we're delivering a reliable plugin, and that we've communicated our intent to other developers as executable descriptions of plugin behavior.
+
+## Building and Testing
+
+Compile and test the plugin with the command:
+
+* `mvn clean verify`
+
+Compile the plugin without running tests using the command:
+
+* `mvn clean -DskipTests verify`
+
+Code coverage reporting is available as a maven target.
+Please try to improve code coverage with tests when you submit.
+
+* `mvn -P enable-jacoco clean install jacoco:report` to report code coverage
+
+Please don't introduce new spotbugs output.
+
+* `mvn spotbugs:check` to analyze project using https://spotbugs.github.io/[Spotbugs].
+* `mvn spotbugs:gui` to review Spotbugs report using GUI
+
+## Automated Tests
+
+Automated tests are run as part of the `verify` phase.
+Run automated tests with multiple Java virtual machines with the command:
+
+```
+$ mvn clean -DforkCount=1C verify
+```
+
+## Issue Reports
+
+Please report issues and enhancements through the [Jenkins issue tracker](https://www.jenkins.io/participate/report-issue/redirect/#15592).