protobuf-codegen-pure docs contradict Cargo book

[elshize]

As mentioned in #324, build scripts are not supposed to write to src directory (https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script), yet this is what the documentation suggests. Once #324 is resolved, I suggest that the documentation should be changed.

[stepancheg]

A lot changed since then, especially in rust-protobuf version 3. Please reopen if the issue is still relevant.