doc (File type association missing by default)

[dderjoel]

Hi thanks for the extension.

Two things that could be improved in the documentation,
there is a typo 'nstall vrom VSIX...'
But more importantly, the fileassociation from jinc to Jasmin was not enabled by default on my machine.
Maybe add a note that after installation (and reload the window) open a jinc file, find the 'Plain Text'-button in the status bar on the right, click it, Choose Configure File Association for 'jinc' and choose Jasmin

Cheers