Add your own precompiles guide (#152)

precompiles/README.md

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+# How to add and test your own precompile
+
+> **DISCLAIMER**: This guide assumes you’re developing in the `zksync_era_precompiles` repo, if not, step one should be done differently.
+
+
+1. Create a file in `precompiles/`.
+2. Add the precompile name to the refresh precompiles script in the `era-test-node` submodule `submodules/era-test-node/scripts/refresh_precompiles.sh`
+3. Wiring the precompile to the node
+    1. Assign an address to it in `submodules/era-test-node/src/deps/system_contracts.rs`.
+    2. Add it to the `yul_contracts` array inside `COMPILED_IN_SYSTEM_CONTRACTS`  in `submodules/era-test-node/src/deps/system_contracts.rs`.
+4. Now, running `make run` should run the node with your added precompile.