Contributing (#19)

* Create CONTRIBUTING.md

* updated contributing guide

CONTRIBUTING.md

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+# Contribution Guidelines for TCD XIOS Demonstration
+
+For code contributions, please create your own Fork of this repository and
+propose code via Pull Request, as detailed in 
+[repositories guidance.](repositories.md)
+
+All code contributions will require a code review prior to merging to `main`.
+
+Pull Requests should test your code contributions with respect to several
+versions of XIOS, and provide a report within the PR. All tests must pass
+for merging to be allowed.
+Please ensure that all new directories have an `__init__.py` so that they
+are 'discovered' by Python's `unittest` and suitable tests of the contributed code.
+
+Please provide a README (or amend an existing one if appropriate) that describes
+the behaviour that is being demonstrated by the contribution.
+
+## Code Contributors
+
+The following people have contributed to this code under the terms of
+the Contributor Licence Agreement and Certificate of Origin detailed
+below:
+
+<!-- start-shortlog -->
+ - Iva Kavcic (Met Office, UK)
+ - Mark Hedley (Met Office, UK)
+ - Harry Shepherd (Met Office, UK)
+<!-- end-shortlog -->
+
+(All contributors are identifiable with email addresses in the version control
+logs or otherwise.)
+
+## Contributor Licence Agreement and Certificate of Origin
+
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I have
+    the right to submit it, either on my behalf or on behalf of my
+    employer, under the terms and conditions as described by this file;
+    or
+
+(b) The contribution is based upon previous work that, to the best of
+    my knowledge, is covered under an appropriate licence and I have
+    the right or permission from the copyright owner under that licence
+    to submit that work with modifications, whether created in whole or
+    in part by me, under the terms and conditions as described by
+    this file; or
+
+(c) The contribution was provided directly to me by some other person
+    who certified (a) or (b) and I have not modified it.
+
+(d) I understand and agree that this project and the contribution
+    are public and that a record of the contribution (including my
+    name and email address) is retained for the full term of
+    the copyright and may be redistributed consistent with this project
+    or the licence(s) involved.
+
+(e) I, or my employer, grant to the UK Met Office and all recipients of
+    this software a perpetual, worldwide, non-exclusive, no-charge,
+    royalty-free, irrevocable copyright licence to reproduce, modify,
+    prepare derivative works of, publicly display, publicly perform,
+    sub-licence, and distribute this contribution and such modifications
+    and derivative works consistent with this project or the licence(s)
+    involved or other appropriate open source licence(s) specified by
+    the project and approved by the
+    [Open Source Initiative (OSI)](http://www.opensource.org/).
+
+(f) If I become aware of anything that would make any of the above
+    inaccurate, in any way, I will let the UK Met Office know as soon as
+    I become aware.
+
+(This Contributor Licence Agreement and Certificate of Origin is
+inspired by the Certificate of Origin used by Enyo and the Linux
+Kernel.)