hydra.el (defhydra): Document the :column feature

Fixes #307

README.md

@@ -427,3 +427,4 @@ Here's a list of body keys that can be overridden in each head:
 - `:exit`
 - `:color`
 - `:bind`
+- `:column`

hydra-examples.el

@@ -333,6 +333,13 @@ _y_: ?y? year       _q_: quit           _L__l__c_: log = ?l?"
 ;; Recommended binding:
 ;; (define-key org-agenda-mode-map "v" 'hydra-org-agenda-view/body)
 
+;;** Example 13: automatic columns
+(defhydra hydra-movement ()
+  ("j" next-line "down" :column "Vertical")
+  ("k" previous-line "up")
+  ("l" forward-char "forward" :column "Horizontal")
+  ("h" backward-char "back"))
+
 ;;* Helpers
 (require 'windmove)
 

hydra.el

@@ -1182,7 +1182,7 @@ nil.  If you don't even want the KEY to be printed, set HINT
 explicitly to nil.
 
 The heads inherit their PLIST from BODY-PLIST and are allowed to
-override some keys.  The keys recognized are :exit and :bind.
+override some keys.  The keys recognized are :exit, :bind, and :column.
 :exit can be:
 
 - nil (default): this head will continue the Hydra state.
@@ -1192,6 +1192,8 @@ override some keys.  The keys recognized are :exit and :bind.
 - nil: this head will not be bound in BODY-MAP.
 - a lambda taking KEY and CMD used to bind a head.
 
+:column is a string that sets the column for all subsequent heads.
+
 It is possible to omit both BODY-MAP and BODY-KEY if you don't
 want to bind anything.  In that case, typically you will bind the
 generated NAME/body command.  This command is also the return