Fix some small issues in the README

[christoph-neumann]

I fixed a couple of issues in the docs that I ran into when I was trying to use the library for the first time. I'm also happy to add instructions for making lein-protobuf work, if you'd like that too.

[astrashe]

Thanks! I just ran into the same issues, and they tripped me up a bit. I don't know about anyone else, but I'd be psyched to read your lein-protobuf instructions.

[DenisKibe]

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