Merge pull request #70 from st0012/patch-1

Use official documentation source and add CONTRIBUTING.md

Author: hsbt

CONTRIBUTING.md

@@ -0,0 +1,3 @@
+## Build and test
+
+Please follow the ["Making Changes To Standard Libraries"](https://docs.ruby-lang.org/en/master/contributing/making_changes_to_stdlibs_md.html) guideline.

README.md

@@ -31,7 +31,7 @@ FileUtils.pwd
 # => "/usr/bin"
 ```
 
-You can find a full method list in the [documentation](https://ruby-doc.org/stdlib/libdoc/fileutils/rdoc/FileUtils.html).
+You can find a full method list in the [documentation](https://docs.ruby-lang.org/en/master/FileUtils.html).
 
 ## Contributing