Update contributing guide

Author: sagikazarmark

.gitattributes

@@ -7,8 +7,7 @@ tests/           export-ignore
 .scrutinizer.yml export-ignore
 .styleci.yml     export-ignore
 .travis.yml      export-ignore
-CONDUCT.md       export-ignore
-CONTRIBUTING.md  export-ignore
+CONTRIBUTING     export-ignore
 phpspec.yml.ci   export-ignore
 phpspec.yml.dist export-ignore
 phpunit.xml.dist export-ignore

CONDUCT.md

@@ -0,0 +1 @@
+Please see http://docs.php-http.org/en/latest/development/contributing.html

CONTRIBUTING

@@ -44,7 +44,7 @@ $ composer test
 
 ## Contributing
 
-Please see [CONTRIBUTING](CONTRIBUTING.md) and [CONDUCT](CONDUCT.md) for details.
+Please see our [contributing guide](http://docs.php-http.org/en/latest/development/contributing.html).
 
 
 ## Security