doc: skeleton KERNEL documentation.

On how to submit new kernels. This should allow a broad community review.

doc/KERNEL.md

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+# Kernels
+
+## Available OpenCL kernels
+
+See directory `kernel`.
+
+
+## Submitting new kernels
+
+### Requirements
+
+TODO
+
+
+### Procedure
+
+1. Copy the kernel you wish to modify and commit it verbatim.
+
+This way, it is easy to verify that there are no hidden changes. Note in
+the commit message which kernel is used as a base.
+
+2. Make changes to the kernel. Commit them.
+
+This way, it can be easily shown what was changed.
+
+3. Search for KL_CKOLIVAS and CKOLIVAS_KERNNAME in the top-level source
+directory and make additions to the listed files in order to integrate
+the new kernel.
+
+Now it can be selected when starting via the `--kernel` argument or
+`kernel` configuration option.
+
+4. Add yourself to the "kernels" section in `AUTHORS.md`. Keep it short.
+
+5. Submit a pull request on GitHub, or file it at the issue tracker,
+listing your git repository and branch name. The current repository and
+issue tracker links should be in `README.md`.