Mention extension in changelog and man page

CHANGELOG.md

@@ -6,6 +6,10 @@
 - SC2245: Warn that Ksh ignores all but the first glob result in `[`
 - SC2243/SC2244: Suggest using explicit -n for `[ $foo ]`
 
+### Changed
+- If a directive or shebang is not specified, a `.bash/.bats/.dash/.ksh`
+  extension will be used to infer the shell type when present.
+
 ## v0.6.0 - 2018-12-02
 ### Added
 - Command line option --severity/-S for filtering by minimum severity

shellcheck.1.md

@@ -64,8 +64,8 @@ not warn at all, as `ksh` supports decimals in arithmetic contexts.
 **-s**\ *shell*,\ **--shell=***shell*
 
 :   Specify Bourne shell dialect. Valid values are *sh*, *bash*, *dash* and *ksh*.
-    The default is to use the file's shebang, or *bash* if the target shell
-    can't be determined.
+    The default is to deduce the shell from the file's `shell` directive,
+    shebang, or `.bash/.bats/.dash/.ksh` extension, in that order.
 
 **-V**,\ **--version**