README: Add information about Drake's fork

[EricCousineau-TRI]

This includes a process for merging PRs.

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This change is 

[EricCousineau-TRI]

+@jwnimmer-tri and +@jamiesnape for review, please?

Reviewable status: 0 of 2 files reviewed, all discussions resolved (waiting on @jamiesnape and @jwnimmer-tri)

[jwnimmer-tri]

Good so far. I'll test out the recipes soon.

Reviewed 2 of 2 files at r1.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @EricCousineau-TRI and @jamiesnape)

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README_DRAKE.md, line 23 at r1 (raw file):

* [User: Python Bindings](https://drake.mit.edu/python_bindings.html#using-the-python-bindings)
* [Dev: Python Bindings](https://drake.mit.edu/doxygen_cxx/group__python__bindings.html)
* [User: API Reference](https://drake.mit.edu/pydrake/index.html#://)

nit I don't think we need the duck emoticon (#://) at the end here?

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README_DRAKE.md, line 60 at r1 (raw file):

Windows testing (with AppVeyor) is disabled for this repository.

## Local Git Setup

Somewhere in this README, I'd like to see a description of any branches and tags that exist on RobotLocomotion/pybind11 that are not already mentioned in the document.

(This would be easier if we purge all of the vestigial branches and tags that have no more reason to exist.)

[jwnimmer-tri]

pending discussions.

Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @EricCousineau-TRI and @jamiesnape)

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README_DRAKE.md, line 69 at r1 (raw file):

    git clone --origin upstream https://github.com/pybind/pybind11
    cd pybind11
    git remote --set-url --push upstream no_push

nit Just set-url not --set-url. Ditto a few lines below.

[EricCousineau-TRI]

Reviewable status: 1 of 2 files reviewed, 1 unresolved discussion (waiting on @jamiesnape and @jwnimmer-tri)

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README_DRAKE.md, line 60 at r1 (raw file):

Previously, jwnimmer-tri (Jeremy Nimmer) wrote…

Somewhere in this README, I'd like to see a description of any branches and tags that exist on RobotLocomotion/pybind11 that are not already mentioned in the document.

(This would be easier if we purge all of the vestigial branches and tags that have no more reason to exist.)

Done.

I'll go ahead and purge the following branches / tags, unless you object:
upstream, upstream_new, last_sha_of_previous_fork (branch and tag)

[EricCousineau-TRI]

Reviewable status: 1 of 2 files reviewed, 1 unresolved discussion (waiting on @jamiesnape and @jwnimmer-tri)

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README_DRAKE.md, line 60 at r1 (raw file):

Previously, EricCousineau-TRI (Eric Cousineau) wrote…

Done.

I'll go ahead and purge the following branches / tags, unless you object:
upstream, upstream_new, last_sha_of_previous_fork (branch and tag)

Oh, and master. I'll set drake to the default branch after this merges.

[jwnimmer-tri]

Reviewed 1 of 1 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @jamiesnape)

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README_DRAKE.md, line 60 at r1 (raw file):

Previously, EricCousineau-TRI (Eric Cousineau) wrote…

Oh, and master. I'll set drake to the default branch after this merges.

Fine by me. (Dunno if you have to consult anyone else?)

[EricCousineau-TRI]

Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @jamiesnape)

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README_DRAKE.md, line 60 at r1 (raw file):

Previously, jwnimmer-tri (Jeremy Nimmer) wrote…

Fine by me. (Dunno if you have to consult anyone else?)

OK I've already asked Jamie about re-setting some branches, so should be good to go.

[EricCousineau-TRI]

-@jamiesnape for now, can fix things afterwards

Reviewable status: complete! all files reviewed, all discussions resolved