Add documentation for the plc_wrapper API

[esonghori]

The source code for the plc_wrapper binary includes many internal Google IPs and that is why it is not open-sourced. An example on how to interact with plc_wrapper can be found in plc_client_test.py. However, a complete documentation for its API is needed.

[Yangxiaojun1230]

Yes, any documents for netlist format would be helpful. And there is a question how do you define the weight of the edge? Noticed that some of the edges have weights and others do not.

################ node { name: "Grp_145/Poutput_single_8" input: "Grp_561/Pinput" attr { key: "macro_name" value { placeholder: "Grp_145" } } attr { key: "type" value { placeholder: "macro_pin" } } attr { key: "weight" value { f: 12 } } ##############