Add README

Signed-off-by: kanpov <[email protected]>

README.md

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+## fctools guest agents
+
+This repository contains two guest agent servers that serve over virtio-vsock, the first using plain HTTP/1.1 and the second using gRPC with HTTP/2.
+
+These guest agents are used in the test suite for `fctools` to ensure the functionality of the HTTP-over-vsock and gRPC-over-vsock extensions.
+
+The HTTP guest agent contains a single POST `/ping` route that accepts JSON and returns JSON, while the gRPC guest agent contains 4 ping-like methods:
+unary, client streaming, server streaming and duplex streaming.
+
+Prebuilt binaries of both guest agents are available on the releases page and in the `testdata` package of `fctools`. Compiling them from source is
+easy as well, run `cargo build -r` in the workspace after ensuring that a stable Rust toolchain is installed alongside `protoc` and the
+`x86_64-unknown-linux-musl` target.