Add docs for detailed add new benchmark in vSwarm

[lrq619]

Compared with original add_benchmark.md, this is a step-by-step guideline on how to implement a new benchmark into vSwarm

[dhschall]

First of all thank you very much for your help on improving the documentation! This is awesome!
I made some notes. Please feel free to include them.

[dhschall]

It does not have to be Greeter. I.e. most of the standalone benchmarks have custom protocols. (https://github.com/vhive-serverless/vSwarm-proto/blob/main/proto/aes/aes.proto). This is actualy the reason why we have the relay. Otherwise, the invoker can be used to invoke the function without the relay.

Suggested change

In `{bench}.proto` file, always define your service as `Greeter` since this is the grpc service that will be invoked by the relay.

In `{bench}.proto` file, define the needed grpc service. Note that invoker and most of vHive work with the simple [HelloWorld](https://grpc.io/docs/what-is-grpc/introduction/) service. However, you can implement arbitrary services for your function as the relay, sitting in between the invoker and your function, can be used to make translations. Refer to [aes.proto](https://github.com/vhive-serverless/vSwarm-proto/blob/main/proto/aes/aes.proto) and the corresponding [translation](https://github.com/vhive-serverless/vSwarm-proto/blob/main/grpcclient/aes_client.go) as an example of a custom service.

[dhschall]

Suggested change

	This generates protobuf files for go
	This generates protobuf files for go.
	Alternatively, run `make proto-all` in the root of vSwarm-proto, which does both steps for all protocols automatically.

[dhschall]

Suggested change

	Then, in `vSwarm-proto/grpcclient` foler, add a file named `{bench}_client.go`, this is the client that relay would return to the invoker. Please refer to other `*_client.go` on how to write this file.
	Then, in `vSwarm-proto/grpcclient` folder, add a file named `{bench}_client.go`. This is the client that the relay will select to invoke your function. Each client must implement [GrpcClient](https://github.com/vhive-serverless/vSwarm-proto/blob/70781f2339083f8c6d298ce0be13e7b7869c1409/grpcclient/grpcclient.go#L79) interface.
	`Init` and `Close` are used to set up and close the connection to the function.
	`GetGenerator` will must return an generator for creating new input values for your function.
	`Request` is the actual invocation of the function.
	Refer to other `*_client.go` clients for more details about how to implement those functions.

[lrq619]

I've take all suggestions and fix some typos.

[ustiugov]

@dhschall can we merge this one?

[dhschall]

Thanks and sorry for the delay