Hum, for what i know, mariadb share the same plugin api of mysql (i didn't found new plugins api at least) but about docs, here have some informations:
it's 5.7, and mariadb is porting mysql 5.6 yet, maybe some features aren't enabled, but if you look mysql 5.5 i think it's fully portable, must check...
the code from mysql to mariadb (plugins) is easly ported in most cases, sometimes you need to change var types but that was the 'biggest' change that i saw (in udf function in my case)