docs: simplify instructions in README
docs: simplify instructions in README

While preparing this:

https://github.com/brianchandotcom/liferay-portal/pull/75811

I realized that the instructions in the README here were wrong: you
don't need to extend "liferay/portal" explicitly ever; instead you get
it by default thanks to liferay-npm-scripts. This is true even when you
extend "liferay/react" or "liferay/metal" in liferay-portal -- you wind
up with a merged list of "liferay/portal" plus "liferay/react" (or
"liferay/metal").

So, I de-emphasized the "liferay/portal" section by moving it down, and
I noted that it is used automatically.

I also simplified the ".eslintrc.js" examples by removing the empty
"rules" sections, because we don't want to actively encourage overriding
in any case.

eduardolundgren/eslint-config-liferay