Merge pull request #727 from nellh/723-documentation-fix

Fix bad developer instructions for npm global install

Author: nellh

README.md

@@ -235,7 +235,7 @@ see available options.
 To develop locally, clone the project and run `yarn` from the project
 root. This will install external dependencies. If you wish to install
 `bids-validator` globally (so that you can run it in other folders), use the
-following command to install it globally: `npm install -g`
+following command to install it globally: `cd bids-validator && npm install -g`
 
 #### Running Locally in a Browser