Rename 'master' branch to 'main'

Author: carlosabalde

README.rst

@@ -440,7 +440,7 @@ INSTALLATION
 
 The source tree is based on autotools to configure the building, and does also have the necessary bits in place to do functional unit tests using the varnishtest tool.
 
-**Beware this project contains multiples branches (master, 4.1, etc.). Please, select the branch to be used depending on your Varnish Cache version (Varnish trunk → master, Varnish 4.1.x → 4.1, etc.).**
+**Beware this project contains multiples branches (main, 4.1, etc.). Please, select the branch to be used depending on your Varnish Cache version (Varnish trunk → main, Varnish 4.1.x → 4.1, etc.).**
 
 Dependencies: