Add --no-root flag to installation command in README

[snehankekre]

Closes #39. Since rags cannot be installed as a package, the installation command needs to include the --no-root flag, which this PR adds.

When absent, running poetry install --with dev leads to the following error:

Writing lock file

Installing the current project: rags (0.0.2)
The current project could not be installed: No file/folder found for package rags
If you do not want to install the current project use --no-root

[jerryjliu]

thanks!