add hint for using constraint file to prevent unwanted updates

[joshbode]

I've found using a constraints.txt file useful for locking the version for packages that I don't want upgraded even when there are new versions.

I think it would be useful to document that "feature" in the README.
(however, it might even be useful to add an option (e.g. pip-review --constraint FILE ...) to specify constraints directly rather than relying indirectly on the behaviour of pip install ...)

[jgonggrijp]

I'm OK with merging this, but please move it further down to not break the overview of pip-review's own CLI. I suggest inserting it after Run ``pip-review -h`` for a complete overview of the options.

Honestly, if you have specific packages that you want to protect against updates beyond some version, you're probably better off using pip-tools with a requirements.in.

[joshbode]

Thanks - I've moved the section as requested.
I'm not familiar with pip-tools and requirements.in - I'll take a look :)