This a simple tutorial but took me a while to figure out owing to the unavailability of proper documentation. So, I thought of documenting it here with the hope that it will save you some pain. Note this tutorial was tested on MacOS High Sierra with a PyCharm Community Edition 2017.1.3
Step 1: Install Markdown Support for PyCharm
Go to Preferences -> Plugins -> Browse Plugins
and search for "markdown" and install the Markdown Support plugin (refer to figure below)
After installation, PyCharm will restart to register the installation.
Step 2: Make sure *.md file pattern is registered to markdown files
Go to Preferences -> Editor -> File Types
and add *.md file pattern for markdown files (refer to figure below).
Reopen your markdown file or restart PyCharm to see the change. You should see something like the following when it is working correctly: