Add usage documentation to README, fixed bugs found while writing

README.md

@@ -1,3 +1,47 @@
 # yamlfmt
 
-A command line tool to format yaml files in your project.
+`yamlfmt` is an extensible command line tool or library to format yaml files. 
+
+## Goals
+
+* Create a command line yaml formatting tool that is easy to distribute (single binary)
+* Make it simple to extend with new custom formatters
+* Enable alternative use as a library, providing a foundation for users to create a tool that meets specific needs 
+
+## Installation
+
+To download the `yamlfmt` command, you can download the desired binary from releases or install the module directly:
+```
+go install github.com/google/yamlfmt/cmd/yamlfmt@latest
+```
+
+## Usage
+
+By default, the tool will recursively find all files that match the glob path `**/*.{yaml,yml}` extension and attempt to format them with the [basic formatter](formatters/basic). To run the tool with all default settings, simply run the command with no arguments:
+```
+yamlfmt
+```
+You can also run the command with paths to each individual file, or with glob paths:
+```
+yamlfmt x.yaml y.yaml config/**/*.yaml
+```
+(NOTE: Glob paths are implemented using the [doublestar](https://github.com/bmatcuk/doublestar) package, which is far more flexible than Go's glob implementation. See the doublestar docs for more details.)
+
+If you would like to have a consistent configuration for include and exclude paths, you can also use a configuration file. The tool will attempt to read a configuration file named `.yamlfmt` in the directory the tool is run on. In it, you can configure paths to include and exclude, for example:
+```
+include:
+- config/**/*.{yaml,yml}
+exclude:
+- excluded/**/*.yaml
+```
+
+The CLI supports 3 operation modes:
+
+* Format (default)
+    - Format and write the matched files
+* Dry run (`-dry` flag)
+    - Format the matched files and output the diff to `stdout`
+* Lint (`-lint` flag)
+    - Format the matched files and output the diff to `stdout`, exits with status 1 if there are any differences
+
+(NOTE: If providing paths as command line arguments, the flags must be specified before any paths)

cmd/yamlfmt/main.go

@@ -15,6 +15,7 @@
 package main
 
 import (
+	"errors"
 	"flag"
 	"log"
 	"os"
@@ -55,7 +56,11 @@ func run() error {
 
 	configData, err := readDefaultConfigFile()
 	if err != nil {
-		return err
+		if errors.Is(err, os.ErrNotExist) {
+			configData = map[string]interface{}{}
+		} else {
+			return err
+		}
 	}
 
 	if len(flag.Args()) > 0 {

command/command.go

@@ -34,7 +34,7 @@ func RunCommand(
 		return err
 	}
 	if len(config.Include) == 0 {
-		config.Include = []string{"**/*.yaml", "**/*.yml"}
+		config.Include = []string{"**/*.{yaml,yml}"}
 	}
 
 	var formatter yamlfmt.Formatter