doc: add project inspect cli example

fix #658

Author: HerringtonDarkholme

website/guide/project/project-config.md

@@ -39,4 +39,32 @@ We can also use directories in `node_modules` to reuse preconfigured rules publi
 More broadly speaking, any git hosted projects can be imported as rule sets by using [`git submodule`](https://www.git-scm.com/book/en/v2/Git-Tools-Submodules).
 :::
 
-##
+## Project Discovery
+
+ast-grep will try to find the `sgconfig.yml` file in the current working directory. If it is not found, it will traverse up the directory tree until it finds one. You can also specify the path to the configuration file using the `--config` option.
+
+```bash
+ast-grep scan --config path/to/config.yml
+```
+
+:::tip Global Configuration
+You can put an `sgconfig.yml` in your home directory to set global configurations for `ast-grep`. XDG configuration directory is **NOT** supported yet.
+:::
+
+Project file discovery and `--config` option are also effective in the `ast-grep run` command. So you can use configurations like [custom languages](/reference/sgconfig.html#customlanguages) and [language globs](/reference/sgconfig.html#languageglobs). Note that `run` command does not require a `sgconfig.yml` file and will stil search code without it, but `scan` command will report an error if project config is not found.
+
+## Project Inspection
+
+You can use the [`--inspect summary`](/reference/cli/scan.html#inspect-granularity) flag to see the project directory ast-grep is using.
+
+```bash
+ast-grep scan --inspect summary
+```
+
+It will print the project directory and the configuration file path.
+
+```bash
+sg: summary|project: isProject=true,projectDir=/path/to/project
+```
+
+Output format can be found in the [GitHub issue](https://github.com/ast-grep/ast-grep/issues/1574).