HACKING: Add necessary commands to configure git send-email

This is convenient for people not involved with projects using email to
as review process.
diff --git a/HACKING b/HACKING
index a8fb403..28f1009 100644
--- a/HACKING
+++ b/HACKING
@@ -108,6 +108,19 @@
 welcome! In order to ease the inclusion of your patch, it's important to follow
 some rules, otherwise it will likely be rejected by maintainers.
 
+Make sure the author name and email are set properly:
+
+  # git config --global user.name <name>
+  # git config --global user.email <email>
+
+The preferred way to send patches is by email, using git send-email:
+
+  # git config --global sendemail.smtpencryption <tls>
+  # git config --global sendemail.smtpserver <smtp.gmail.com>
+  # git config --global sendemail.smtpuser <yourname@gmail.com>
+  # git config --global sendemail.smtpserverport <587>
+  # git config sendemail.to linux-bluetooth@vger.kernel.org
+
 BlueZ rules for submitting patches follow most of the rules used by Linux kernel
 (https://www.kernel.org/doc/Documentation/SubmittingPatches) with some remarks:
 
@@ -128,3 +141,17 @@
 should be limited to 50 characters and the description should be wrapped at 72
 characters except if it contains quoted information from debug tools like
 backtraces, compiler errors, etc.
+
+6) Prefix the email subject with [PATCH BlueZ]:
+
+  # git config format.subjectprefix "PATCH BlueZ"
+
+6) Add a cover letter when introducing a new feature explaning what problem
+you're trying to solve:
+
+  # git format-patch --cover-letter -M origin/master -o outgoing/
+  # edit outgoing/0000-*
+
+7) Submit:
+
+  # git send-email outgoing/*