add example setting environment variable

Author: adamint

TESTING.md

@@ -1,10 +1,12 @@
 # Testing
 
-We use [Spek](https://github.com/spekframework/spek) to run unit tests. You must add Maven Central to the gradle repositories 
-in order to pull Spek.
+We use [Spek](https://github.com/spekframework/spek) to run unit tests. 
 
 To run any test, you must have two environment variables, `SPOTIFY_CLIENT_ID` and `SPOTIFY_CLIENT_SECRET` set to a Spotify application in your current shell.
 
+`export SPOTIFY_CLIENT_ID=your_client_id`
+`export SPOTIFY_CLIENT_SECRET=your_client_secret`
+
 To run **only** public endpoint and utility tests, run `gradle check`
 
 To run **all** tests, you need a valid Spotify redirect uri, and token (string). These are `SPOTIFY_REDIRECT_URI` and `SPOTIFY_TOKEN_STRING` respectively.