Update README.md w/ small but important detail

[cristian-spiescu]

E.g. https://github.com/HugoHasenbein/redmine_more_previews#use

w/ something like:

In order to use this plugin, you need to explicitly enable it per project. I.e. `for a project > Settings (tab) > Project (tab, the first one) > Modules (field set/box) > Check "Redmine More Previews"`

I felt into this trap. Initially I though the code was not working. Browsing a bit through the issues: I saw this often.

[batrachian]

Good catch! When did that change happen?

[cristian-spiescu]

I think it's from the beginning so. It would be interesting from somebody w/ write access on the repo, to apply this change.

[batrachian]

Yes, I think you must be correct about that. One very nice thing about redmine is once it's set up it requires (for me at least) very little administration and it's been so long for me that I completely forgot. Just got caught out after migrating a redmine site.

Yes a note would be helpful.
And possibly a fast way of enabling it on a large number of projects, although that can be done by modifying the database directly, too.