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
}
}
##############