Merge pull request #9 from stuart-warren/docs

docs: update docs to include usage

README.md

@@ -1,8 +1,21 @@
 # yamlfmt
 
+[![Go build](https://github.com/stuart-warren/yamlfmt/workflows/Go/badge.svg)](https://github.com/stuart-warren/yamlfmt/actions)
+
 based on gofmt, yamlfmt formats yaml files into a canonical format
 
 * lists are not indented
 * maps have sorted keys
 * indent is 2 spaces
 * documents always have separators `---`
+
+```
+$ yamlfmt --help
+formats yaml files with 2 space indent, sorted dicts and non-indented lists
+usage: yamlfmt [flags] [path ...]
+  -d    display diffs instead of rewriting files
+  -l    list files whose formatting differs from yamlfmt's
+  -w    write result to (source) file instead of stdout
+```
+
+Without an explicit path, it processes the standard input. Given a file, it operates on that file; given a directory, it operates on all .yaml and .yml files in that directory, recursively. (Files starting with a period are ignored.) By default, yamlfmt prints the reformatted sources to standard output.