Update README.md

Author: sebastiencs

README.md

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+Various binaries used during witness generation, and testings
+
+- All `*_gates.json` are the list of gates, for each circuit
+- `*_internal_vars.bin` and `*_rows_rev.bin` are other data we need for witness generation: [link](https://github.com/MinaProtocol/mina/blob/28ad2335c3970b856849137196c7693c18edee78/src/lib/crypto/kimchi_backend/common/plonk_constraint_system.ml#L707-L710)
+- All others `*.bin` are snark works (`ExternalSnarkWorkerRequest`) used for testings
+- `*.txt` files are witnesses vectors generated from OCaml, they are used for testings too