added .dvmrc version lock support

[Chathula]

Story

This PR Incudes,

• added .dvmrc file for deno version lock. this will automatically pick deno version from .dvmrc when run dvm use
• updated the readme file with usage

[axetroy]

Thanks for your PR, but I do n’t think it ’s a good idea to add a .dvmrc

Keep it as simple as possible

[Chathula]

Thanks for your PR, but I do n’t think it ’s a good idea to add a .dvmrc

Keep it as simple as possible

i think it is something important to have as nvm has it. this is something nodejs devs use daily to prevent from braking changes mainly. it will be important in the future when deno releases more breaking changes.

and most of the nodejs CIs use nvmrc when build the artifacts. deno will need it. just consider it again.

Thanks