feat(boojum): Adding README to prover directory (#189)

# What ❔

* Adding README to prover directory to describe the high level layout of
the code

## Why ❔

* to improve the knowledge and understanding of prover components

prover/README.md

@@ -0,0 +1,50 @@
+# Prover directory
+
+This directory contains all the libraries and binaries related to proving of the blocks.
+
+Directories with 'fri' suffix, are mostly used with the new proof system (Boojum).
+
+## Components
+
+### vk_setup_data_generator_server_fri
+
+Set of tools to create setup keys, verification keys and verification key hashes for the circuits.
+
+Usually run once, and then we use their outputs in multiple places in the system.
+
+### prover_fri_gateway
+
+Communication module between the 'main' server running the state keeper, and the proving subsystem.
+
+### witness_generator
+
+Creating prover jobs and saving necessary artifacts.
+
+### prover_fri
+
+This directory contains the main 'prover'. It can be run in two modes: either as CPU or as GPU. (controlled via 'gpu'
+feature flag).
+
+### witness_vector_generator
+
+Only used in GPU proving mode. Prepares all the witness data using CPU, and then streams it to the prover_fri.
+
+This is mosty used for resource efficiency (as machines with GPUs are more expensive, it allows us to run many
+witness_vector_generators, that can 'share' as single gpu based prover_fri).
+
+### proof_fri_compressor
+
+Used as a 'last step' to compress/wrap the final FRI proof into a SNARK (to make L1 verification cheaper).
+
+## Old proof system
+
+Some of the components here belong to the old proof system:
+
+- circuit_synthesizer
+- prover
+- setup_key_generator_and_server
+
+Moreover old proof system is also using components from 'core' directory, like:
+
+- core/bin/verification_key_generator_and_server
+- core/lib/zksycn_core/src/witness_generator.