Add additional ExecConfig template documentation

[nabertrand]

The sample spp.conf.yml has some examples, but maybe there could also be links to the actual engines/parsers used with the templates.

For example:

GithubWebhooks/Listeners/ExecConfig Args/SuccessMessage/ErrorMessage use oliveagle/jsonpath for "request" values

GenericExecTasks Args/SuccessMessage/ErrorMessage use http/Request.Form (url.Values) for "request" values

Each of the templates above is rendered using html/template