added 'next_rails' exe to support cli option for version

[rishijain]

Closes #15

Description:

There are a few executables or command-line options already present for example bundle_report, deprecations, next,
but none of them really reflects the whole gem as such.

So I have added a new executable called next_rails and we can use it like next_rails --version.
This is a general convention followed by libraries that provide options like version,
for ex: puma gem provides puma --version as a command-line option.

I have also updated the READme so the documentation covers the mention of this new option.

I will abide by the code of conduct.