The trend is to drop the "e" before adding the suffix -able. The two terms you cite have both forms and only knowledgeable is more common than knowledgable, probably because it is an old coinage.
As the living suffix, -able is useful for coining new words, though we often have to ignore spell check when it comes to -able coinages. For example, our spell check disapproves of sanctionable, channelable, overthrowable, redoable, and torturable, but these are perfectly good words and do not require hyphenation.
To form an -able word, treat the verb as you do when making an -ing participle. For example, we make moving from move by dropping the e and adding -ing. So, to make move‘s -able adjective, we drop the e and add -able: movable.
knowledgeable (adj.) also knowledgable, (Ngram):
- c. 1600, "capable of being known, recognizable" (a 17c. sense now obsolete), from knowledge in its Middle English verbal sense + -able. The sense of "having knowledge, displaying mental capacity" is from 1829 and probably a new formation.
also likable, (Ngram):
- 1730, a hybrid from like (v.) + -able. Related: Likeableness. Middle English had likeworthy (from Old English licwyrðe "agreeable, acceptable").