Update README.md (#98)

When using policybot with a custom server, I initially expected the behavior described to be:

* The custom path is checked
* If the custom path does not exist, the default path is checked

In actuality, if a custom path is configured, *only* the custom path is checked. This update attempts to call that out more clearly.

README.md

@@ -39,8 +39,9 @@ UI to view the detailed approval status of any pull request.
 ## Configuration
 
 By default, the behavior of the bot is configured by a `.policy.yml` file at
-the root of the repository. The file name and location are configurable when
-running your own instance of the server.
+the root of the repository. When running your own instance of the server, a
+different file name and location can be configured. The configured name and
+location will be used instead of the default location.
 
 - If the file does not exist, the `policy-bot` status check is not posted. This
   means it is safe to enable `policy-bot` on all repositories in an organization.