Update mkdocs-material instructions in README. Fixes PagerDuty#25 (Pa…

…gerDuty#26)

README.md

@@ -9,7 +9,7 @@ You can view the documentation [directly](/docs/index.md) in this repository, or
 We use [MkDocs](http://www.mkdocs.org/) to create a static site from this repository. For local development,
 
 1. [Install MkDocs](http://www.mkdocs.org/#installation). `pip install mkdocs`
-1. Install the [MkDocs Material theme](https://github.com/squidfunk/mkdocs-material). `pip install mkdocs-material`
+1. Install v0.2.4 of the [MkDocs Material theme](https://github.com/squidfunk/mkdocs-material). `pip install mkdocs-material==0.2.4`
 1. To test locally, run `mkdocs serve` from the project directory.
 
 ## Deploying