Add kernelhacking mailing list

Signed-off-by: Hao Lee <haolee.swjtu@gmail.com>

README.md

@@ -7,6 +7,24 @@ A book-in-progress about the linux kernel and its insides.
 
 **Questions/Suggestions**: Feel free about any questions or suggestions by pinging me at twitter [@0xAX](https://twitter.com/0xAX), adding an [issue](https://github.com/0xAX/linux-insides/issues/new) or just drop me an [email](mailto:anotherworldofworld@gmail.com).
 
+# Mailing List
+
+We have a Google Group mailing list for learning the kernel source code. Here are some instructions about how to use it.
+
+#### Join
+
+Send an email with any subject/content to `kernelhacking+subscribe@googlegroups.com`. Then you will receive a confirmation email. Reply it with any content and then you are done.
+
+> If you have Google account, you can also open the [archive page](https://groups.google.com/forum/#!forum/kernelhacking) and click **Apply to join group**. You will be approved automatically.
+
+#### Send emails to mailing list
+
+Just send emails to `kernelhacking@googlegroups.com`. The basic usage is the same as other mailing lists powered by mailman.
+
+#### Archives
+
+https://groups.google.com/forum/#!forum/kernelhacking
+
 Support
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