Add doc comment for exported Format func (#115)

stack.go

diff --git a/stack.go b/stack.go
index 6b1f289..cbe3f3e 100644
--- a/stack.go
+++ b/stack.go
@@ -79,6 +79,14 @@
 // StackTrace is stack of Frames from innermost (newest) to outermost (oldest).
 type StackTrace []Frame
 
+// Format formats the stack of Frames according to the fmt.Formatter interface.
+//
+//    %s	lists source files for each Frame in the stack
+//    %v	lists the source file and line number for each Frame in the stack
+//
+// Format accepts flags that alter the printing of some verbs, as follows:
+//
+//    %+v   Prints filename, function, and line number for each Frame in the stack.
 func (st StackTrace) Format(s fmt.State, verb rune) {
 	switch verb {
 	case 'v':