Enable to open the .md file from Open in Studio Lab button

**Is your feature request related to a problem? Please describe.**
A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]

Open in Studio Lab button can open the `.ipynb` file but can not open the `.md` file.

https://github.com/aws/studio-lab-examples/issues/11

**Describe the solution you'd like**

Open in Studio Lab button can open the `.md` file.
When the students push the `Open in Studio Lab` button on the GitHub repository of the lecture materials like [Machine-Learning-with-Python](https://github.com/tirthajyoti/Machine-Learning-with-Python), they can open the `README.md` that is top of the repository.

**Describe alternatives you've considered**
To direct some `.ipynb` file in the repository.

**Additional context**
(none)