Add a small section to readme with reference to InstantOn doc and guide

[leochr]

Add a small section to readme with reference to InstantOn doc and guide

• Completed for WL
• Completed for OL

[kabicin]

Added small section to the readme's below:
OL: https://github.com/OpenLiberty/ci.docker/pull/689/files
WL: https://github.com/WASdev/ci.docker/pull/738/files