Add documentation and changelog

Author: TrySound

CHANGELOG.md

@@ -41,6 +41,9 @@ postcssImport({
 })
 ```
 
+- Changed: support promise in `transform` option and `undefined` result will be skipped
+([#147](https://github.com/postcss/postcss-import/pull/147))
+
 # 7.1.3 - 2015-11-05
 
 - Fixed: ensure node 0.12 compatibility, round 2

README.md

@@ -109,7 +109,8 @@ _Note: nested `@import` will additionally benefit of the relative dirname of imp
 Type: `Function`  
 Default: `null`
 
-A function to transform the content of imported files. Take one argument (file content) & should return the modified content.
+A function to transform the content of imported files. Take one argument (file content) and should return the modified content or promise with it.
+`undefined` result will be skipped.
 
 #### `plugins`