mock: add short readme (#502)

mock/README.md

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+# Miden mock
+
+Utilities to help test Miden programs.
+
+This crate contains builder and mock functions to create objects for testing. A
+mock chain which allows the simulation of a rollup in-memory. And some precompute
+values to speed up testing.
+
+## Features
+
+| name         | description                                                                                                       |
+| ------------ | ----------------------------------------------------------------------------------------------------------------- |
+| `std`        | Allows usage of Rust's `std`, for `no-std` compile with `--no-default-features`.                                  |
+| `serde`      | Enable `serde` based seralization of most objects. This feature allows saving the mock chains state to a file.    |
+| `executable` | Enables the creation of `mock` executable, used to save mock data to a file, which can later on be used by tests. |
+
+## License
+
+This project is [MIT licensed](../LICENSE).