docs: add readme

README.md

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+# supa-mdx-lint
+
+Work in progress.
+
+An MDX linter meant to enforce the Supabase Docs style guide.
+
+## Usage
+
+```
+Usage: supa-mdx-lint [OPTIONS] <TARGET>
+
+Arguments:
+  <TARGET>  (Glob of) files or directories to lint
+
+Options:
+  -c, --config <FILE>    Sets a custom config file
+  -f, --fix              Auto-fix any fixable errors
+      --format <FORMAT>  Output format [default: simple]
+  -d, --debug            Turn debugging information on
+  -s, --silent           Do not write anything to the output
+  -h, --help             Print help
+  -V, --version          Print version
+```
+
+## Configuration
+
+The default configuration file is `supa-mdx-lint.config.toml`, relative to the
+root of your working directory. You can point to a different config file using
+the `--config` option.
+
+Use the config file to define ignore patterns:
+
+```
+ignore_patterns = []
+```
+
+Or configure rule-specific settings:
+
+```
+[Rule001HeadingCase]
+may_uppercase = []
+```
+
+Or configure rule error levels:
+
+```
+[Rule001HeadingCase]
+level = "warn"
+```
+
+Or turn rules off entirely:
+
+```
+Rule001HeadingCase = false
+```