README enhancement : "Usage" section only applies to Unix/Linux

[TonyGravagno]

From the README:

Usage: JupyterLab and Jupyter Notebook
After you register tslab to Jupyter, start JupyterLab and Jupyter Notebook as usual. You can now create JavaScript and TypeScript notebooks.

That is followed by the CLI for either jupyter lab or jupyter notebook as executed from shell. However, it's written like a single script to be executed and it's not clear that this is for Unix/Linux only. A Windows user will almost never execute it like that.

Suggested text enhancement:

You can now create JavaScript and TypeScript notebooks. Unix/Linux users will execute one of the following two commands, depending on the platform. Windows users should have a shortcut, and probably will only use these commands from scripts.