Fix Echo example and improve instructions

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`

.gitignore

@@ -1,3 +1,5 @@
 .DS_Store
 project.xcworkspace
 xcuserdata
+.build
+third_party/**