docs: add Require documentation

README.md

@@ -165,6 +165,7 @@ ssh vhs.example.com < demo.tape > demo.gif
 There are a few basic types of VHS commands:
 
 * [`Output <path>`](#output): specify file output
+* [`Require <program>`](#require): specify required programs for tape file
 * [`Set <Setting> Value`](#settings): set recording settings
 * [`Type "<characters>"`](#type): emulate typing
 * [`Left`](#arrow-keys) [`Right`](#arrow-keys) [`Up`](#arrow-keys) [`Down`](#arrow-keys): arrow keys
@@ -187,6 +188,21 @@ Output out.webm
 Output frames/ # a directory of frames as a PNG sequence
 ```
 
+### Require
+
+The `Require` command allows you to specify dependencies for your tape file.
+These are useful to fail early if a required program is missing from the
+`$PATH` and it is certain that the VHS execution will not work as expected.
+
+Require commands must be defined at the top of a tape file, before any non-
+setting or non-output command.
+
+```elixir
+# A tape file that requires gum and glow to be in the $PATH
+Require gum
+Require glow
+```
+
 ### Settings
 
 The `Set` command allows you to change global aspects of the terminal, such as

man.go

@@ -25,6 +25,7 @@ A tape file is a script made up of commands describing what actions to perform i
 The following is a list of all possible commands in VHS:
 
 * %Output% <path>.(gif|webm|mp4)
+* %Require% <program>
 * %Set% <setting> <value>
 * %Sleep% <time>
 * %Type% "<string>"