ARROW-2907: [GitHub] Improve the first paragraph of "How to contribut…

…e patches"

In previous version, it is unclear that contributors need to follow
the procedure "To contribute a patch". In this version, it clearly
indicate that contributors need to follow the procedure described
later in the section.

Author: Kenji Okimoto <okimoto@clear-code.com>

Closes #2319 from okkez/improve-contributing and squashes the following commits:

7b9d7f5 <Kenji Okimoto> ARROW-2907:  Improve the first paragraph of "How to contribute patches"

.github/CONTRIBUTING.md

@@ -37,7 +37,8 @@ to end up in JIRA, either before or after completing a pull request.
 ## How to contribute patches
 
 We prefer to receive contributions in the form of GitHub pull requests. Please
-send pull requests against the [github.com/apache/arrow][4] repository.
+send pull requests against the [github.com/apache/arrow][4] repository following
+the procedure below.
 
 If you are looking for some ideas on what to contribute, check out the [JIRA
 issues][3] for the Apache Arrow project. Comment on the issue and/or contact
@@ -68,4 +69,4 @@ Thank you in advance for your contributions!
 [1]: mailto:dev-subscribe@arrow.apache.org
 [2]: https://github.com/apache/arrow/tree/master/format
 [3]: https://issues.apache.org/jira/browse/ARROW
-[4]: https://github.com/apache/arrow
+[4]: https://github.com/apache/arrow