add usage

.travis.yml

@@ -19,8 +19,5 @@ cache:
 before_install:
   - echo "TRAVIS_GO_VERSION=${TRAVIS_GO_VERSION}"
 
-install:
-  - go mod install
-
 script:
   - go test -v

README.md

@@ -4,3 +4,14 @@
 [![GoDoc](https://godoc.org/github.com/yi-ge/unxz?status.svg)](https://godoc.org/github.com/yi-ge/unxz)
 [![codebeat badge](https://codebeat.co/badges/1a06ccdf-73a1-4e8c-bf84-b40c8f6beeb2)](https://codebeat.co/projects/github-com-yi-ge-unxz-master)
 [![Go Report Card](https://goreportcard.com/badge/github.com/yi-ge/unxz)](https://goreportcard.com/report/github.com/yi-ge/unxz)
+
+Golang \*.tar.xz decompress.
+
+## Usage
+
+```golang
+import "github.com/yi-ge/unxz"
+
+u := unxz.New(filePath, outDir)
+err := u.Extract()
+```