[readme]: clarify that lts/* points to the active LTS

[paulirish]

IMO https://nodejs.org/en/about/releases/ could do a better job clarifying that "current" is a non-LTS classification

but regardless.. lts/* is basically like a lts/active .. but since that other page is kinda weak on distinguishing "current" from "active lts", i think this small addition helps. :)

jordan if you think i should propose some wording changes to /releases i'm happy to followup with that, as well.

[ljharb]

Hi Paul!

"active" implies a support date, and lts/* locally will point to whatever version you have installed that is the latest version in the latest known LTS line.

If the last time you ran nvm ls-remote or nvm install, Argon was the latest release line (which is now EOL), and you have v4.9.1 installed, then lts/* and lts/argon will both point to it.

"active" or "inactive" aren't concepts nvm captures, in other words - just "everything" or "LTS" (which current isn't).

I'm happy to clarify that the release schedule's "Current" is not considered LTS, but i don't want to get into whether a line is active or not, or supported or not.