update README

Author: uchiiii

README.md

@@ -1,5 +1,18 @@
 # CCompiler
 compiler for C language
 
+# environment
+Use docker with `Dockerfile`
+
+## build
+```
+$ make 9cc
+```
+
+## test
+```
+$ make test
+```
+
 ## Thanks
 I referred to 9cc in [this article](https://www.sigbus.info/compilerbook). Thus, I thank [Rui Ueyama](https://twitter.com/rui314) to share his knowledge.