Merge pull request react-bootstrap#434 from RichardLitt/docs/add-npm-install-line-433

docs: Added instructions for npm install in main dir

Author: mtscout6

docs/README.md

@@ -16,6 +16,8 @@ $ cd react-bootstrap/docs
 $ npm install
 ```
 
+You must have previously run `npm install` from the main directory. 
+
 ### Instructions
 
 #### Development