Add usage instruction.

README.md

@@ -1 +1,22 @@
-# docker-goby
+# docker-goby
+
+## Usage
+
+### Pull Image
+
+```
+$ docker pull st0012/goby:latest
+```
+
+### Execute File
+
+```
+# In container, default working directory is /go/src/github.com/goby-lang/goby 
+$ docker run -t -v PATH_TO_TARGET_FILE_DIR:/go/src/github.com/goby-lang/goby st0012/goby:latest goby TARGET_FILE
+```
+
+For example if I want to run `./samples/loop.gb`, I will run
+
+```
+$ docker run -t -v `pwd`/samples:/go/src/github.com/goby-lang/goby st0012/goby:latest goby loop.gb
+```