Minor UNITTEST readme tweaks

- Add link to troubleshooting section at the top of the page
- Update GCC requirements for Windows users

UNITTESTS/README.md

@@ -1,12 +1,12 @@
 # Unit testing Mbed OS
 
-This document describes how to run and write unit tests for Mbed OS.
+This document describes how to run and write unit tests for Mbed OS. Please see the [troubleshooting](#troubleshooting) section if you run into any issues.
 
 ## Prerequisites
 
 * GNU toolchains installed.
     * GCC 6 or later
-    * MinGW-W64 GCC-6.4.0 or MinGW-W64 GCC-7.3.0 (Windows)
+    	* MinGW-W64 is recommended, although any windows port of the above GCC versions should work
 * CMake 3.0+ installed.
 * Python 2.7.x or >3.5 and pip 10.0 (or newer) installed.
 * gcovr >=4.1
@@ -291,4 +291,4 @@ TEST_F(TestSemaphore, constructor)
 
 **Problem:** virus protection identifies files generated by CMake as malicious and quarantines the files on Windows.
 
-* **Solution**: restore the false positive files from the quarantine.
+* **Solution**: restore the false positive files from the quarantine.