Update CONTRIBUTING.md

Closes Homebrew/homebrew-cask#28354.

CONTRIBUTING.md

@@ -1,3 +1,22 @@
 # How to Contribute
 
 All of the [instructions for contributing from the main repository](https://github.com/caskroom/homebrew-cask/blob/master/CONTRIBUTING.md) apply, and won't be repeated here. You simply need to change references to `caskroom/homebrew-cask` (that repository) to `caskroom/homebrew-versions` (this repository).
+
+# Nightly Builds (Nightlies)
+
+For Casks that use nightlies, which are generally builds that are done once a day, the `version`, `sha256` and `url` will change frequently and hence the Cask will be constantly outdated. To alleviate having to regularly update a Cask for a nightly build, the use of a [`url do` block](https://github.com/caskroom/homebrew-cask/blob/master/doc/cask_language_reference/stanzas/url.md#using-a-block-to-defer-code-execution) in conjunction with `version :latest` and `sha256 :no_check` can help ensure that the latest version is always used.
+
+See [this pull request for exist-db-nightly](https://github.com/caskroom/homebrew-versions/pull/3067) for an example of the procedure.
+
+Example ([exist-db-nightly.rb](https://github.com/caskroom/homebrew-versions/blob/16b3bab91ab5b9a69ef7c456441b0e0fced56516/Casks/exist-db-nightly.rb#L6#L14)):
+```ruby
+  url do
+    require 'open-uri'
+    base_url = 'http://static.adamretter.org.uk/exist-nightly'
+    builds_url = "#{base_url}/table.html"
+    latest_build_filename = open(builds_url) do |io|
+      io.read.scan(%r{<tr>.*?<td>(.*?)</td>.*?<a href="([^\"]+)">dmg}m).max[1]
+    end
+    "#{base_url}/#{latest_build_filename}"
+  end
+```