Update changelog

Author: Gianluca Arbezzano

CHANGELOG.md

@@ -1,3 +1,8 @@
+Release 0.3.0
+* [#18](https://github.com/vim-php/vim-composer/pull/18) Added missing ComposerUpdate function
+* [#21](https://github.com/vim-php/vim-composer/pull/21) Added missing CONTRIBUTING.md file
+* [#20](https://github.com/vim-php/vim-composer/pull/20) Require and/or init commands
+
 Release 0.2.0
 * [#17](https://github.com/vim-php/vim-composer/pull/17) Improved ComposerKnowWhereCurrentFileIs and readme file
 * [#16](https://github.com/vim-php/vim-composer/pull/16) Local variable are now global in ComposerKnowWhereCurrentFileIs