add contributing guidelines

CONTRIBUTING.md

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+# Contributing to wis2-gdc
+
+We welcome contributions to wis2-gdc, in the form of issues, bug fixes, or
+suggestions for enhancements. This document sets out our guidelines and best
+practices for such contributions.
+
+It's based on the [Contributing to Open Source Projects
+Guide](https://contribution-guide-org.readthedocs.io/).
+
+wis2-gdc has the following modes of contribution:
+
+- GitHub Commit Access
+- GitHub Pull Requests
+
+## Code of Conduct
+
+Contributors to this project are expected to act respectfully toward others in accordance with the [Code of Conduct](https://github.com/wmo-im/wis2-gdc/blob/main/CODE_OF_CONDUCT.md).
+
+## Submitting Bugs
+
+### Due Diligence
+
+Before submitting a bug, please do the following:
+
+* Perform __basic troubleshooting__ steps:
+
+    * __Make sure you're on the latest version.__ If you're not on the most
+      recent version, your problem may have been solved already! Upgrading is
+      always the best first step.
+    * [__Search the issue
+      tracker__](https://github.com/wmo-im/wis2-gdc/issues)
+      to make sure it's not a known issue.
+
+### What to put in your bug report
+
+Make sure your report gets the attention it deserves: bug reports with missing
+information may be ignored or punted back to you, delaying a fix.  The below
+constitutes a bare minimum; more info is almost always better:
+
+* __What version of Python are you using?__ For example, are you using Python
+  2.7, Python 3.7, PyPy 2.0?
+* __What operating system are you using?__ Windows (7, 8, 10, 32-bit, 64-bit),
+  Mac OS X,  (10.7.4, 10.9.0), GNU/Linux (which distribution, which version?)
+  Again, more detail is better.
+* __Which version or versions of the software are you using?__ Ideally, you've
+  followed the advice above and are on the latest version, but please confirm
+  this.
+* __How can the we recreate your problem?__ Imagine that we have never used
+  wis2-gdc before and have downloaded it for the first time. Exactly what steps
+  do we need to take to reproduce your problem?
+
+## Contributions and Licensing
+
+### Contributor License Agreement
+
+Your contribution will be under our [license](https://github.com/wmo-im/wis2-gdc/blob/main/LICENSE) as per [GitHub's terms of service](https://help.github.com/articles/github-terms-of-service/#6-contributions-under-repository-license).
+
+### GitHub Commit Access
+
+* Proposals to provide developers with GitHub commit access shall be emailed to the wis2-gdc [team](TODO). Proposals shall be approved by the wis2-gdc development team.  Committers shall be added by the project admin.
+* Removal of commit access shall be handled in the same manner.
+
+### GitHub Pull Requests
+
+* Pull requests may include copyright in the source code header by the contributor if the contribution is significant or the contributor wants to claim copyright on their contribution.
+* All contributors shall be listed at https://github.com/wmo-im/wis2-gdc/graphs/contributors
+* Unclaimed copyright, by default, is assigned to the main copyright holders as specified in https://github.com/wmo-im/wis2-gdc/blob/main/LICENSE
+
+### Version Control Branching
+
+* Always __make a new branch__ for your work, no matter how small. This makes
+  it easy for others to take just that one set of changes from your repository,
+  in case you have multiple unrelated changes floating around.
+
+    * __Don't submit unrelated changes in the same branch/pull request!__ If it
+      is not possible to review your changes quickly and easily, we may reject
+      your request.
+
+* __Base your new branch off of the appropriate branch__ on the main repository:
+
+    * In general the released version of wis2-gdc is based on the ``main``
+      (default) branch whereas development work is done under other non-default
+      branches. Unless you are sure that your issue affects a non-default
+      branch, __base your branch off the ``main`` one__.
+
+* Note that depending on how long it takes for the dev team to merge your
+  patch, the copy of ``main`` you worked off of may get out of date!
+    * If you find yourself 'bumping' a pull request that's been sidelined for a
+      while, __make sure you rebase or merge to latest ``main/``__ to ensure a
+        speedier resolution.
+
+### Code Formatting
+
+* __Please follow the coding conventions and style used in the wis2-gdc repository.__
+* wis2-gdc endeavours to follow the
+  [PEP-8](http://www.python.org/dev/peps/pep-0008/) guidelines.
+
+## Suggesting Enhancements
+
+We welcome suggestions for enhancements, but reserve the right to reject them
+if they do not follow future plans for wis2-gdc.