Update README.md

[mahoutukaisali]

SUMMARY

Added a note of extra_settings parameters to warn users that those parameters cannot change after deploying awx.
This is critical information because if users need to change those parameters after deploying awx, there is only one way that
they need to rebuild instances.

ISSUE TYPE

• Bug, Docs Fix or other nominal change

[TheRealHaoLiu]

@mahoutukaisali changing the extra_settings is allowed after awx is deployed, changing extra_settings will cause update to the deployment of awx pods which causes a rolling restart of awx.

can you elaborate on what you mean by "extra_settings cannot change after deployed awx"

[mahoutukaisali]

@TheRealHaoLiu I mean, values configured in extra_settings cannot change from Web UI.