[Docs] Add squash guide and apache contributors guide link (apache#4221)

Co-authored-by: wangfan <[email protected]>

CONTRIBUTING.md

@@ -5,6 +5,10 @@ We want to have high quality, well documented codes for each programming languag
 
 Nor is code the only way to contribute to the project. We strongly value documentation, integration with other project, and gladly accept improvements for these aspects.
 
+Recommend reading:
+ * [Contributors Tech Guide](http://www.apache.org/dev/contributors)
+ * [Get involved!](http://www.apache.org/foundation/getinvolved.html)
+
 ## Contributing code
 
 To submit a change for inclusion, please do the following:
@@ -13,6 +17,19 @@ To submit a change for inclusion, please do the following:
 #### If you are introducing a completely new feature or API it is a good idea to start a [RIP](https://github.com/apache/rocketmq/wiki/RocketMQ-Improvement-Proposal) and get consensus on the basic design first.
 #### It is our job to follow up on patches in a timely fashion. Nag us if we aren't doing our job (sometimes we drop things).
 
+### Squash commits
+
+If your have a pull request on GitHub, and updated more than once, it's better to squash all commits.
+
+1. Identify how many commits you made since you began: ``git log``.
+2. Squash these commits by N: ``git rebase -i HEAD~N`` .
+3. Leave "pick" tag in the first line.
+4. Change all other commits from "pick" to "fixup".
+5. Then do "force push" to overwrite remote history: ``git push -u origin ROCKETMQ-9999 --force``
+6. All your changes are now in a single commit, that would be much better for review.
+
+More details of squash can be found at [stackoverflow](https://stackoverflow.com/questions/5189560/squash-my-last-x-commits-together-using-git).
+
 ## Becoming a Committer
 
 We are always interested in adding new contributors. What we look for are series of contributions, good taste and ongoing interest in the project. If you are interested in becoming a committer, please let one of the existing committers know and they can help you walk through the process.
@@ -25,7 +42,7 @@ Nowadays,we have several important contribution points:
 ##### Prerequisite
 If you want to contribute the above listing points, you must abide our some prerequisites:
 
-###### Readability - API must have Javadoc,some very important methods also must have javadoc
+###### Readability - API must have Javadoc, some very important methods also must have javadoc
 ###### Testability - 80% above unit test coverage about main process
 ###### Maintainability - Comply with our [checkstyle spec](style/rmq_checkstyle.xml), and at least 3 month update frequency
 ###### Deployability - We encourage you to deploy into [maven repository](http://search.maven.org/)