Output android versionname as string

I have my versionCode and versionName set in my manifest.xml

android:versionCode="2"
android:versionName="Alpha 1.2"

But how do I output this as a string so the user can see what version of the app they have?

In my "about" fragment I attempted to use:

android:text="?android:attr/versionName"

but this didnt seem to work. It literally put "?android:attr/versionName" as the output string.