Rebuild proto artifacts from existing sources and update README.md instructions.

[mikelehen]

This is the result of me updating README.md and following the steps in it. Some of the diffs were for a proto change Greg made a while ago. I'm not really sure why Firestore.pbrpc.h and Firestore.pbrpc.m changed. Everything seems to still build, but if you suspect I did something wrong, let me know. I've never done this before.

I'm doing this in preparation for /actually/ updating the protos.

[wilhuff]

The spurious diff you've encountered comes from the fact that the generation strategy for imports changed. It was released with gRPC 1.8.0, which went live 12/12/2017.

I last generated these files before our initial release which was merged into this repo in bde743e on 10/3/2017. I attempted to regenerate this in January in bc74670, but rolled it back after the change didn't work as expected. That change included these diffs.

It's likely that when @rsgowman was working in here on adding nanopb support he just reverted these diffs as being unrelated?

Basically: I don't see anything wrong with this.

[mikelehen]

Thanks! If you don't see anything wrong, could you approve it then? :-) Or is there something extra I should do first? [I'll have another PR that actually updates the protos, but it'll be good to get these diffs checked in separately I think...]

[wilhuff]

LGTM (doh)