common: added a helpers directory with common shell helpers.

common/helpers/README

@@ -0,0 +1,25 @@
+SHELL HELPERS
+=============
+
+This directory contains shell files that are read by xbps-src and can be used at any
+phase when building source packages. Only files with the `.sh' extension will be read.
+
+A shell helper must provide its own function for use in the source packages.
+The following examples illustrates a fake helper named `myhelper.sh' that provides
+the `myhelper_func' function:
+
+myhelper.sh:
+...
+myhelper_func() {
+	...
+}
+...
+
+You can then use this helper in a source package like this:
+
+srcpkgs/foo/template:
+...
+do_install() {
+	myhelper_func ...
+}
+...