Front page documentation additions

The term 'stream' in programming can means lots of different things and
'activity' doesn't narror it down much so we try to clarify the exact
purpose of the project.

Provide explicit example of Github activity stream as a common usage and
one that user's will probably be familiar with.

Explicity mention support of Django models to make it a little clearer
for beginners.

Add 'Github repository' to activity stream example to clarify a little
as without it an 'activity-stream' is quite an abstract concept which is
pretty general in the context of the project but knowing that we're
actutally talking about the repository or project as a real life object
is useful.
Provide basic run down of urls

Otherwise it is a little confusing as to why there would be urls to
install as the project generally feels like it is useful but abstract
underlying framework rather than something that might provide something
as concreate as urls.
Reduce & move warning and clarify Manager role

When I first read this page my attention was drawn entirely to the
warning, missing the very important paragraph above it. The warning is
only relevant to older more familiar users who would be looking for an
explanation of behaviour change and possibly find it in the change log
anyway.

I've change it to a note and moved it down to push focus towards the
main text.

Provided some extra sentences about the Manager. Many Django users won't
be familiar with the term 'Manager' as you can do alot with just the
MyModel.objects manager that you can by default without thinking of it
as anything other than 'that thing I used for queries'.

Clarify that you can only have one activity-stream manager per app with
the current configuration set up.
Expand on generating actions documentation

Link to the Django signals documentation.

Break part the second example and be a little more explicit about what
'group' and 'comment' might be in this example.

Describe a little of how to think about the generated activities and
where they are stored and how they are queried. I feel it is important
to help users build a picture of the usage and limitations of the
system.
Merge pull request #195 from michaeljones/docs

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