add max_function_clause_length to rules.md

RULES.md

@@ -26,6 +26,7 @@ identified with `(since ...)` for convenience purposes.
 - [Max Anonymous Function Arity](doc_rules/elvis_style/max_anonymous_function_arity.md)
 - [Max Function Arity](doc_rules/elvis_style/max_function_arity.md)
 - [Max Function Length](doc_rules/elvis_style/max_function_length.md)
+- [Max Function Clause Length](doc_rules/elvis_style/max_function_clause_length.md)
 - [Max Module Length](doc_rules/elvis_style/max_module_length.md)
 - [Module Naming Convention](doc_rules/elvis_style/module_naming_convention.md)
 - [Nesting Level](doc_rules/elvis_style/nesting_level.md)

doc_rules/elvis_style/max_function_clause_length.md

@@ -0,0 +1,27 @@
+# Max Function Clause Length
+
+This specifies an upper bound on function clause **line** length. Lines that are comments and/or whitespace
+can be either included or excluded from the line count.
+
+> Works on `.beam` file? Not really! (it consumes results Ok, but these might be unexpected, since
+the files are pre-processed)
+
+## Options
+
+- `max_length :: non_neg_integer()`.
+  - default: `30`.
+- `count_comments :: boolean()`
+  - default: `false`
+- `count_whitespace :: boolean()`
+  - default: `false`
+
+## Example
+
+```erlang
+{elvis_style, max_function_clause_length}
+%% or
+{elvis_style, max_function_clause_length, #{ max_length => 30
+                                    , count_comments => false
+                                    , count_whitespace => false
+                                    }}
+```