Create the extending clink page

[sleepyhollo]

There is a link to extending clink in its website and in the github readme. Both these links lead to clink.md page which is TODO.

However, the clink installation folder has a clink.html file which has a completed section on Extending clink. I haven't read that part of the document yet, but I think that should go in the clink.md document.

[chrisant996]

Martin moved the old content from clink.md to a readme.md in the root of the repo as part of starting to rewrite the documentation because of the extensive changes in 1.0.0a1. But he disappeared before updating the documentation. I'll update the documentation in the chrisant996/clink fork when it eventually starts to get near to a release.