Add an example host to demonstrate gdbstub

hosts/host-gdbstub/README.md

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+# Why?
+
+To debug your guest application. GDB can attach to the host over TCP socket, stdio,
+serial connection, etc. In the provided example, it communicate over stdio, however
+using `socat` it can be redirected to a TCP socket.
+
+# How to use it
+
+1. Compile a guest application, for example from [apps](../../apps/) directory.
+   To have a better debugging experience, use `-O0` and `-g3`.
+
+2. Run the host prividing the app binary file. Use the following command to
+   redirect stdio to a TCP socket:
+
+```
+socat -v -x
+      TCP-LISTEN:1234,reuseaddr \
+      EXEC:"./host-gdbstub ../../apps/maze/maze.bin"
+```
+
+`-v` and `-x` are used to see the traffic in hex.
+
+3. Fire up a `riscv32-elf-gdb` (might be different name on your OS) and give
+   it the generated elf file from step 1:
+
+`riscv32-elf-gdb -ex 'target remote :1234' -ex 'layout src' maze.elf`
+
+4. Happy debugging. `s`, `si`, `n`, breakpoint, etc should be working :)
+