Documentation: which RF variables are supported in the manager when resolving the path of resource files

[damies13]

Update documentation to note which RF variables are supported in the manager when resolving the path of resource files

E.g: ${CURDIR} And not: ${EXECDIR}

From Issue #170 :

When I dig into the code that deals with the RF variables (function is replace_rf_path_variables)

I see the comment

	# ${EXECDIR}
	# not sure how to handle this for now

I realised it's never handled ${EXECDIR} as I remember now writing that comment when I wrote that function.

That line in v1.2.0 ->

https://github.com/damies13/rfswarm/blob/1921386410c32926e342f18000d48b623e1935ad/rfswarm_manager/rfswarm.py#L1261

[damies13]

Also need to mention to use relative paths