add transient.md

Author: vindarel

content/en/usage/transient.md

@@ -0,0 +1,37 @@
+---
+title: Transient help popup
+weight: -30
+---
+
+A transient is a text pop-up window, displaying at the bottom of the
+screen, that shows keybindings and their associated commands.
+
+The pop-up can stay in place when the user presses a key, which allows
+to always show help on-screen.
+
+## Simple example
+
+```lisp
+  (define-command example-command () ()
+    (message "example message"))
+
+  (lem/transient:define-transient *demo-keymap-simple*
+    :description "my demo keymap"
+    (:key "a" :suffix 'example-command :description "demo prefix 1")
+    (:key "b" :suffix 'example-command :description "demo prefix 2" :active-p nil))
+
+  (define-key *global-keymap* "C-c t" *demo-keymap-simple*)
+```
+
+<div style="text-align: center;"> <img src="https://raw.githubusercontent.com/lem-project/lem/main/extensions/transient/transient-simple.png"/> </div>
+
+## Show all current keybindings
+
+Setting `lem/transient:*transient-always-show*` to `t` makes it show
+all available keybindings for the current mode(s):
+
+```lisp
+(setf lem/transient::*transient-always-show* t)
+```
+
+<div style="text-align: center;"> <img src="https://raw.githubusercontent.com/lem-project/lem/main/extensions/transient/transient-simple.png"/> </div>