Create CONTRIBUTING.md

Proposal for a process to contribute tests

Author: edwardsph

CONTRIBUTING.md

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+# Contributing tests
+
+The following is a proposal for a process by which tests can be contributed. The process will need testing and reviewing
+so is likely to change.
+
+* Add an issue for new tests using the label “Proposed Test Case” - this should reference the specification requirement
+  to be tested and outline how you expect to test that requirement.
+* When someone picks up an issue, add the label “In Progress”.
+* Any discussion on how to implement the test should be captured in the issue.
+* A new test will need the following:
+  * An entry in the relevant manifest file (e.g. in protocol/ or web-access-control/). The initial status should be
+    `td:reviewStatus td:unreviewed`.
+  * If there are comments to make about the requirement, e.g. saying why a particular implementation doesn’t support it,
+    these can be added to the matching requirement-comments.ttl with `rdfs:comment`.
+  * Create the feature file - see notes in the [README](README.md) and refer to other tests as examples of common patterns.
+* Submit a PR and request a review from another contributor (e.g. @edwardsph)
+* The reviewer, at this stage, will focus on the test implementation more than how well it tests the requirement.
+* The test is then merged and will be used by various Solid server implementors. This will provide feedback that will be part
+  of a subsequent review by specification editors. If the test is approved by the editors, the manifest status will change
+  to `td:approved`. Note the process for review by editors is not yet defined.
+
+For advice on writing tests, you are welcome to contact @edwardsph via https://gitter.im/solid/test-suite