Update README for branch choice

This commit updates the README to indicate that the "Get the code, build
it, test it" link will help decide the correct branch to work in.

Also fixes a grammar issue and cleans-up the Markdown a little bit.

README.md

-Code status:
-------------
+# Code status:
 
 * [![Appveyor CI status](https://ci.appveyor.com/api/projects/status/4u6pexmtpuf8jq66?svg=true)](https://ci.appveyor.com/project/rasmushoj/server) ci.appveyor.com
 
-## MariaDB: The innovative open source database 
+## MariaDB: The innovative open source database
 
 MariaDB was designed as a drop-in replacement of MySQL(R) with more
 features, new storage engines, fewer bugs, and better performance.
@@ -33,20 +32,19 @@ https://mariadb.com/kb/en/mariadb-versus-mysql-compatibility/
 
 https://mariadb.com/kb/en/new-and-old-releases/
 
-Getting the code, building it and testing it
----------------------------------------------------------------
+# Getting the code, building it and testing it
 
-Refer to the following guide: https://mariadb.org/get-involved/getting-started-for-developers/get-code-build-test/ which outlines how to correctly build the source code and run the MariaDB testing framework.
+Refer to the following guide: https://mariadb.org/get-involved/getting-started-for-developers/get-code-build-test/
+which outlines how to build the source code correctly and run the MariaDB testing framework,
+as well as which branch to target for your contributions.
 
-Help
------
+# Help
 
 More help is available from the Maria Discuss mailing list
 https://lists.mariadb.org/postorius/lists/discuss.lists.mariadb.org/ and MariaDB's Zulip
-instance, https://mariadb.zulipchat.com/ 
+instance, https://mariadb.zulipchat.com/
 
-Licensing
----------
+# Licensing
 
 ***************************************************************************
 
@@ -60,8 +58,7 @@ license information can be found in the THIRDPARTY file.
 
 ***************************************************************************
 
-Bug Reports
-------------
+# Bug Reports
 
 Bug and/or error reports regarding MariaDB should be submitted at:
 https://jira.mariadb.org