Add a section on suggested version requirements to Application guide

[indirect]

I just had this conversation for the one billionth time, and we should definitely make this a section in the Applications guide so that people can find it by searching, and we can link to it instead of repeating it:

• every single gem that has no version constraint on it means you are telling bundler it is required to consider every single version of that gem that has ever existed
• if you are not okay with Bundler giving you version 0.1.0 of that gem, I recommend adding version constraints
• also the number of possible versions involved increases the resolving time by something like n^2
• so ruling out versions by adding version constraints speeds up the resolving by a huge amount
• my usual pattern is to use rails at a single fully locked version, like “4.2.7.1"
• and use all other gems at a movable tiny version, like “~> 3.2.12”
• some gems that have especially trustworthy releases might get movable minor versions like “~> 3.2"
  -but all of those things combined means bundle install or bundle update GEM can run really quickly