Add hacking guide for SDK developers

Author: moznion

HACKING.md

@@ -0,0 +1,43 @@
+Hacking guide for SDK developers
+==
+
+First of all
+--
+
+Please execute `make install-devtool`.
+
+How to run tests
+--
+
+Use `make test`.
+
+How to execute [PHP_CodeSniffer](https://github.com/squizlabs/PHP_CodeSniffer)
+--
+
+Use `make phpcs`.
+
+How to execute [PHPMD](https://phpmd.org/)
+--
+
+Use `make phpmd`.
+
+How to execute them all
+--
+
+`make`
+
+Release Flow
+--
+
+1. Make a git tag (this project uses semantic versioning)
+1. Push the tag to origin
+
+That's all. It will be publish on [composer](https://packagist.org/packages/linecorp/line-bot-sdk) automatically.
+
+e.g.
+
+```
+$ git tag 1.2.3
+$ git push origin 1.2.3
+```
+

README.md

@@ -184,21 +184,7 @@ Requirements
 For SDK developers
 --
 
-### How to run tests
-
-Use `make test`.
-
-### How to execute [PHP_CodeSniffer](https://github.com/squizlabs/PHP_CodeSniffer)
-
-Use `make phpcs`.
-
-### How to execute [PHPMD](https://phpmd.org/)
-
-Use `make phpmd`.
-
-### How to execute them all
-
-`make`
+Please refer [HACKING.md](/HACKING.md).
 
 See also
 --