Update the README

[mxgrey]

Since rmf_traffic doesn't contain as much content as rmf_core used to, I think we should push the high level README content into the meta rmf repo and just link to that from rmf_traffic.

[codecov[bot]]

Codecov Report

Merging #22 (70167e8) into main (91356f4) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #22   +/-   ##
=======================================
  Coverage   32.66%   32.66%           
=======================================
  Files         155      155           
  Lines       15633    15633           
  Branches    10435    10435           
=======================================
  Hits         5107     5107           
  Misses       1694     1694           
  Partials     8832     8832           

Flag	Coverage Δ
tests	32.66% <ø> (ø)

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[mxgrey]

This resolves #9

[gbiggs]

I think that the correct place for the removed content would be the root page of the documentation in the rmf_docs repository. We can optionally add it to that repository's README as well, if we want, but I don't think that's necessary since the entry page for RMF should be the documentation site, not a GitHub repository.

[mxgrey]

All of the removed content is actually old junk that shouldn't even exist anymore, because it's incorrect (irredeemably out of date).

The most up to date high-level documentation is here (source). I suppose we may want to migrate the information in that "book" into rmf_docs at some point.