More description of calendar identifier retrieval

[nrhinehart]

The README is insufficient for informing the user how to extract the relevant calendar string. I added more information that works for the current google calendar settings interface, and an additional example in the setting to illustrate that calendars can also come in non-email address form.

[telotortium]

Hi everyone! This repository is obsolete - the actively developed fork is now https://github.com/kidd/org-gcal.el, which is also the "org-gcal.el" on MELPA (see #115). I'm one of the maintainers of https://github.com/kidd/org-gcal.el. I can't directly close issues or PRs on this repo, but please download https://github.com/kidd/org-gcal.el and see if your issue or PR is still applicable. If so, open an issue or PR on https://github.com/kidd/org-gcal.el. In any case, you should close your issue or PR in this repo.