Binder dependencies

[jeffjennings]

After removing the global requirements.txt in #606, the Binder build broke because it expects that file. In general it expects a single requirements file for all notebooks (see here; for details of how to specify requirements, see here).

This PR adds binder/requirements.txt to the top-level directory, which Binder uses to successfully build. This single requirements file currently works for all notebooks. If in the future individual notebooks need packages installed with versions that differ from those in binder/requirements.txt, execution of those notebooks in Binder will fail.

PR also updates text in the README.

Closes #615
Closes #616
Closes #535

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[adrn]

I think this approach is fine and practical, but I wonder if we want a mechanism to automatically generate this top-level requirements file based on all of the individual tutorial requirements files?

[adrn]

I guess then there will be ambiguous cases if there are disagreements in version requirements... I think I'm ok with this approach for now (even though there is duplicated information that we have to manually keep in sync) but let's make an issue with the aspirational goal of having a script to "build" the global requirements file automatically.