Merge pull request #6537 from tomjonandy/docs/cask-tap

Docs: Add cask instructions to tap docs

docs/How-to-Create-and-Maintain-a-Tap.md

 # How to Create and Maintain a Tap
 
-[Taps](Taps.md) are external sources of Homebrew formulae and/or external commands. They
-can be created by anyone to provide their own formulae and/or external commands
+[Taps](Taps.md) are external sources of Homebrew formulae, casks  and/or external commands. They
+can be created by anyone to provide their own formulae, casks  and/or external commands
 to any Homebrew user.
 
 ## Creating a tap
@@ -61,6 +61,19 @@ Once your tap is installed, Homebrew will update it each time a user runs
 `brew update`. Outdated formulae will be upgraded when a user runs
 `brew upgrade`, like core formulae.
 
+## Casks
+
+Casks can also be installed from a tap.
+Casks can be included in taps with formulae, or in a tap with just casks.
+Place any cask files you wish to make available in a `Casks` directory at the top level of your tap.
+
+See [homebrew/cask](https://github.com/Homebrew/homebrew-cask) for an example of a tap with a `Casks` subdirectory.
+
+### Naming
+
+Unlike formulae, casks must have globally unique names to avoid clashes.
+This can be achieved by e.g. prepending the cask name with you github username: `username-formula-name`.
+
 ## External commands
 
 You can provide your tap users with custom `brew` commands by adding them in a