Fix Echo example and improve instructions

[acchou]

Added header search path to the Echo example to allow it to build. Also
added instructions to run the script in the third party directory to
clone the swift-protobuf repo locally, which is required by the Echo
example but not mentioned. Also update the toplevel readme to make the
required make command explicit, and fix the name swift-proto to
swift-protobuf

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[acchou]

Signed the CLA.

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CLAs look good, thanks!

[timburks]

Looks good, thanks!