Semantic Versioning (and Pessimistic Versioning Constraint) notice

I noticed some projects using Fog are setting very specific versioning constraint.  For example, https://github.com/mitchellh/vagrant-rackspace/issues/34 happened because both vagrant-aws and vagrant-rackspace are using 3 digits of precision.

The fog RELEASE.md says that Fog follows semantic versioning.  Several other projects have put up notices suggesting to use 2 digits of precision for projects that follow semver.  See https://github.com/intridea/multi_json, for example.

I added a similar notice, but softened the language because I don't think there is a clear understanding on what semantic versioning means for a project like Fog, which has multiple providers backed by services that are all evolving at their own pace.
Merge pull request #2313 from maxlinc/patch-1

Semantic Versioning (and Pessimistic Versioning Constraint) recommendation