Update Docs to Avoid Confusion About Usage of App

Description

A user (that is new to interoperability of djangocms apps) is likely to encounter some issues when following the documentation:

Be unaware how to set "page meta" (#86, #66).
Be unaware how to render "page meta" ("6. That's all!" without mentioning "Usage").
Be unclear of the distinction between "meta"s (comment on djangocms-blog #550):
- "page meta" (django-page-meta)
- "blog article meta" (djangocms-blog using django-meta).
Add both of, or be unsure what to do about, the two "render meta stuff" template markups (one from django-meta and one from djangocms-page-meta (comment on #86).

In the the readme, between the current (as of this issue's creation date) step 5 and step 6 of "Quickstart", add a step that mentions and links to "Usage".
In the usage doc, add mention, for indirect users of django-meta (perhaps via djangocms-blog), that they need not have both of these lines of markup:
- {% include 'djangocms_page_meta/meta.html' with meta=page_meta %}
- {% include "meta/meta.html" %} Only the top one is necessary.
In djangocms-blog docs (via a separate, unwritten, issue), add mention of djangocms-page-meta and describe its purpose and distinction from and relation to django-meta, in the context of djangocms-blog usage.
- Also, possibly, note that <head {% meta_namespaces %}> is still necessary for djangocms-blog usage of django-meta

Google+ is dead. Remove the documentation about it (and create an issue to remove the code)?