Added a contributing guide (openwdl#130)

CONTRIBUTING.md

@@ -0,0 +1,26 @@
+# Contributing to WDL
+
+WDL strives to be community driven and thus relies on suggestions and assistance from people such as yourself. We welcome any contribution that you are willing to provide whether it be in the form of syntax suggestions, feature requests, documentation help, or simply pointing out our ~~tyops~~ typos. All that we ask is that you agree to the following [contributor agreement](#contributor-agreement) and follow a [few guidelines](#how-to-contribute).
+
+# Contributor agreement
+
+I understand that WDL is a community-driven standard. Any ideas or suggestions I provide may be used by the community and may become part of the official WDL specification under the [BSD 3-clause license](https://github.com/broadinstitute/wdl/blob/develop/LICENSE). I attest that all contributions are my own and not copyrighted by another party.
+
+# How to contribute
+## Contributions to WDL discussions
+
+WDL is the bioinformatics workflow language meant to be written and read by humans. In order to achieve this goal we rely on actual humans to help keep us on the right course.  Fellow WDL enthusiasts debate the finer points of syntax over in the [WDL developers forum](FIXME) and everyone is invited to participate. You'll need to register yourself to participate in the forum.
+
+## Contributions to the WDL ecosystem
+
+As WDL increases in popularity, a number of people are [building tools](https://software.broadinstitute.org/wdl/toolkit) to make it easier to interact with WDL or platforms that support it. If you've created something which you think would be useful we'd love to hear about it! We can help you help us help everyone.
+
+## Contributions to the documentation, code, etc
+
+We welcome any contribution to the WDL [Github repository](https://github.com/broadinstitute/wdl), whether it be to the [specification](https://github.com/broadinstitute/wdl/blob/develop/SPEC.md), [example scripts](https://github.com/broadinstitute/wdl/tree/develop/scripts), [syntax highlighters](https://github.com/broadinstitute/wdl/tree/develop/highlighters), or anything we haven't yet thought of. To do this, first fork then clone the repo:
+
+    git clone [email protected]:your-username/wdl.git
+
+Make your change and then push your changes to the fork and [submit a pull request](https://help.github.com/articles/creating-a-pull-request-from-a-fork/)
+
+