Update README.md

Add note describing iOS app store safety.
Add a TaggedVersionNotFound error
Update README.md

Reword and relocate.
Add checks for 0.x SemVer incompatibility
Update comment describing 0.x SemVer treatment
Use a separate error code enum
Merge pull request #200 from Carthage/tagged-version-not-found

Add a TaggedVersionNotFound error
Merge pull request #198 from swizzlr/patch-1

Update README.md
Remove parens because Swift
Add note to Artifacts about SemVer usage with 0.x versions
Merge pull request #202 from eliperkins/feature/0.x-semver

Add checks for 0.x SemVer incompatibility
Merge branch 'master' into 0.4-development

Conflicts:
	CarthageKit/Resolver.swift
	CarthageKit/Version.swift
	CarthageKitTests/VersionSpec.swift
	Documentation/Artifacts.md