start a clippy-dev readme and some rough info on how to use lintcheck

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+## Clippy-dev is a tool to ease clippys development, similar to `rustc`s `x.py`.
+
+Functionalities (incomplete):
+
+# lintcheck
+Runs clippy on a fixed set of crates read from `clippy_dev/lintcheck_crates.toml`
+and saves logs of the lint warnings into the repo.
+We can then check the diff and spot new or disappearing warnings.
+
+From the repo root, run:
+````
+cargo run --target-dir clippy_dev/target --package clippy_dev \
+--bin clippy_dev --manifest-path clippy_dev/Cargo.toml --features lintcheck -- lintcheck
+````
+or
+````
+cargo dev-lintcheck
+````
+
+By default the logs will be saved into `lintcheck-logs/lintcheck_crates_logs.txt`.
+
+You can set a custom sources.toml by adding `--crates-toml custom.toml`
+where `custom.toml` must be a relative path from the repo root.
+
+The results will then be saved to `lintcheck-logs/custom_logs.toml`.
+