Update README.md

Author: wrouesnel

README.md

@@ -42,6 +42,30 @@ Package vendoring is handled with [`govendor`](https://github.com/kardianos/gove
 * `web.telemetry-path`
   Path under which to expose metrics.
 
+### Environment Variables
+
+The following environment variables configure the exporter:
+
+* `DATA_SOURCE_NAME`
+  the default legacy format. Accepts URI form and key=value form arguments. The
+  URI may contain the username and password to connect with.
+
+* `DATA_SOURCE_URI`
+   an alternative to DATA_SOURCE_NAME which exclusively accepts the raw URI
+   without a username and password component.
+
+* `DATA_SOURCE_USER`
+  When using `DATA_SOURCE_URI`, this environment variable is used to specify
+  the username.
+* `DATA_SOURCE_USER_FILE`
+  The same, but reads the username from a file.
+
+* `DATA_SOURCE_PASS`
+  When using `DATA_SOURCE_URI`, this environment variable is used to specify
+  the password to connect with.
+* `DATA_SOURCE_PASS_FILE`
+  The same as above but reads the password from a file.
+
 ### Setting the Postgres server's data source name
 
 The PostgreSQL server's [data source name](http://en.wikipedia.org/wiki/Data_source_name)