Update CONTRIBUTING.md
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
pivotal/LicenseFinder,bdshroyer/LicenseFinder,bdshroyer/LicenseFinder,pivotal/LicenseFinder,tinfoil/LicenseFinder,bspeck/LicenseFinder,tinfoil/LicenseFinder,bspeck/LicenseFinder,sschuberth/LicenseFinder,pivotal/LicenseFinder,tinfoil/LicenseFinder,sschuberth/LicenseFinder,pivotal/LicenseFinder,pivotal/LicenseFinder,bspeck/LicenseFinder,pivotal/LicenseFinder,tinfoil/LicenseFinder,bdshroyer/LicenseFinder,tinfoil/LicenseFinder,sschuberth/LicenseFinder,bspeck/LicenseFinder,bdshroyer/LicenseFinder,bdshroyer/LicenseFinder,sschuberth/LicenseFinder,pivotal/LicenseFinder,bspeck/LicenseFinder,sschuberth/LicenseFinder