LintDiff README

eng/tools/lint-diff/README.md

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+# TODO: REVERIFY THESE DOCS BEFORE MERGE
+# LintDiff
+
+LintDiff looks at files changed in a commit, runs `autorest` linting over those
+files, and looks for "new" errors in the results. 
+
+Altered files examined include `readme.md` files and swagger `.json` files. In
+the case of `readme.md` files the tool will examine tags and other files that
+could be impacted by the change (e.g. `readme.md` files in folders higher in the
+directory structure).
+
+## Run Locally
+
+1. Setup `before` - Clone the specs repo at the *base commit* for the changes you want to test. (generally cloned to main and in a temp location like `/tmp/rest-api-specs-before`)
+1. Setup `after` - Clone the specs repo at the commit for the changes you want to test.
+1. Build a list of changed files from `after` generally by running `git diff --name-only <base> > changed-files.txt` 
+1. From the root of the repo run `npm i` to install dependencies. 
+1. Run `npm exec --no -- lint-diff --before <before> --after <after> --changed-files <changed-files.txt>`
+
+### Example 
+
+In most common cases, changes to LintDiff are in the current state of the repo and you'll need to set up a `before` to run against.
+
+```bash
+git clone https://github.com/Azure/azure-rest-api-specs.git /tmp/rest-api-specs-before
+git diff --name-only main > changed-files.txt
+
+npm i
+npm exec --no -- lint-diff --before /tmp/rest-api-specs-before --after . --changed-files changed-files.txt
+```