README/RUNTESTS.md

[geoffmcl]

As can be seen the RUNTESTS.md could do with some updates, especially for running in unix...

And specifically the run-tests.sh needs to be greatly enhanced to run all test types, with different inputs, outputs, etc, etc...

In the equivalent Windows alltest.bat it supports commands, namely [-t path/to/tidy.exe] [-o output_directory/] [-c case_set_name], and even has a --help...

This issue was suggested in Issue 718

My unix shell scripting skills are just ok, but really seek the help of a person with more experience to assist here... thanks...