CT-143 added global vars documentation in README

Author: colinbut

README.md

@@ -47,6 +47,14 @@ Alternatively, you can check out this example demo project which shows some use
 A lot of the configuration for external systems (Github, SonarQube etc) are set in the `Constants.groovy` constants class. 
 You can edit this to match according to your settings.
 
+### Gloval Vars Documentation
+
+Each of the global variables (helper functions) has its own documentation (corresponding *.txt file). You can access these documentation on the __Global Variable Reference__ link on the navigation sidebar of pipeline jobs that imported this shared library. 
+
+Note that as per Jenkins documentation this is only visible after one successful run of the pipeline job for the documentation to be generated.
+
+The documentation is in (.txt) file format and contains HTML.
+
 ## Release Notes
 
 __v0.0.1__