v0.15.0

Fixes:

• Let shards build error if no targets defined (#490, thanks @straight-shoota)
• Fix to allow empty shard.override.yml (#495, thanks @straight-shoota)
• Stop expecting master to be the default branch for git (#503, thanks @szabgab)

Features:

• Add documentation for shard.override.yml (#494, thanks @straight-shoota)
• Warn only crystal version (#496, thanks @beta-ziliani, @bcardiff)
• Don't default the Crystal version to <1.0.0, use only the lower bound (#493, thanks @oprypin)
• Add --skip-executables (#506, thanks @straight-shoota)

Others:

• Escape automatic ligatures in AsciiDoc (#489, thanks @elebow)
• Fix links in README (#500, #483, thanks @szabgab, @kimburgess)
• Correct list identation in shard.yml.adoc (#492, thanks @elebow)
• Add getting started section to README (#513, thanks @straight-shoota)