Improve documentation for version constraints

[mietek]

• Clarify using HALCYON_CONSTRAINTS or including a cabal.config file is optional.
• Explain the way Halcyon uses cabal.config files. (mietek/halcyon#40 (comment))
• Clarify setting HALCYON_IGNORE_ALL_CONSTRAINTS to 1 does not cause version constraints included in the build-depends field of a Cabal package description file to be ignored. (mietek/halcyon#48 (comment))
• Prepare for upcoming Stackage-related changes. (mietek/halcyon#41)