Document NoDuplicateTags

Author: mpchadwick

README.md

@@ -36,6 +36,7 @@ pre-commit:
     variables: ['description']
   - check: FrontMatterVariableMeetsLengthRequirements
     variables: ['description', 'title']
+  - check: NoDuplicateTags
 ```
 
 ## Available Checks
@@ -76,6 +77,10 @@ For example...
 
 In the above, there would be a maximum length of 50 characters for the title (rather than the default of 59)
 
+#### NoDuplicateTags
+
+Checks that you have not accidentally introduced a duplicate tag. For example, if you have posts that are already using "MySQL" as a tag and you attempt to commit a post with the tag "mysql"  (notice the difference in case-sensitivity) you will not be allowed to commit.
+
 ## Roll Your Own
 
 You can also add your own checks. To do so, create a class in the `Jekyll::PreCommit::Checks` module and and define a `check` method. Your class should extend the `Jekyll::PreCommit::Checks::Check` class and return the `@result` instance variable.