Update CONTRIBUTING.md

Added CocoaPods installation instructions, with an additional detailed description of a potential pitfall with CocoaPod version inconsistencies. CocoaPods feature tests will not pass if the CocoaPods/Specs repo used with version is CocoaPods version 0.39 and below

CONTRIBUTING.md

@@ -80,3 +80,19 @@ JAVA_OPTS='-client -XX:+TieredCompilation -XX:TieredStopAtLevel=1' JRUBY_OPTS='-
 ### Gradle
 
 You'll need a gradle version >= 1.8.
+
+### CocoaPods
+LicenseFinder supports CocoaPods 0.39 and below. If you are using a later version of CocoaPods, you will need to downgrade CocoaPods/Specs repository in order to use LicenseFinder. [This article](http://blog.cocoapods.org/Sharding/) describes the breaking change between CocoaPods 0.39 and 1.0. You will need to use an older, archived CocoaPods/Specs repo. 
+
+If you see the following error, try switching to the archived repo. 
+
+```bash
+[!] The `master` repo requires CocoaPods 1.0.0 -  (currently using 0.34.0)
+ ```
+
+Example of how to switch to the archived repo: 
+
+```bash 
+mv ~/.cocoapods/repos/master ~/.cocoapods/repos/master.bak
+git clone https://github.com/CocoaPods/Old-Specs.git ~/.cocoapods/repos/master
+```