Added information about clay.el

[daslu]

Added information about clay.el, a CIDER extension to support Clay - a tool for data visualization and literate programming.

Replace this placeholder text with a summary of the changes in your PR. The more detailed you are, the better.

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Before submitting the PR make sure the following things have been done (and denote this by checking the relevant checkboxes):

• [x] The commits are consistent with our contribution guidelines
• [ ] You've added tests (if possible) to cover your change(s)
• [ ] All tests are passing (eldev test)
• [ ] All code passes the linter (eldev lint) which is based on elisp-lint and includes
  • byte-compilation, checkdoc, check-declare, packaging metadata, indentation, and trailing whitespace checks.
• [x] You've updated the changelog (if adding/changing user-visible functionality)
• [x] You've updated the user manual (if adding/changing user-visible functionality)

Thanks!

If you're just starting out to hack on CIDER you might find this section of its manual extremely useful.

[daslu]

Thanks!