add declarative pipeline restriction note on useable param names

[gabemontero]

Fixes https://github.com/openshift/jenkins-plugin/issues/160

also adds a note about IS SCMs vs. ICTs with pipeline BCs

@openshift/sig-developer-experience fyi

[gabemontero]

just a readme change, but i'll wait for any input on the verbiage before merging

[bparees]

lgtm