I have already given you the procedure to add additional libs this this answer
As said adding external libs may causes tricky versionning conflict issues that can cause misbehavior of the platform and/or of your custom code.
I think you should rather wait for our analysis on your requirement to convert HTML to Markdown in order to benefit from an out of the box solution.
PS: May I ask what platform version you are using ? I ask because if you are using version 6.2 you should upgrade to 6.3 as no further changes will be done on version 6.2 which will end its short term maintenance period in ~2 months