[xclipboard] Update README.org

A simple edit adding documentation about the various platform-specific programs
this layers interacts with.

CHANGELOG.develop

@@ -2240,6 +2240,8 @@ Other:
 - Avoid loading all the diff packages (thanks to Sylvain Benner)
 - Fixed error on =diff-hl-margin-mode= function being nil during startup (thanks
   to Voleking)
+**** Xclipboard
+- Added Requirements documentation (thanks to Chris Glass)
 **** Ycmd
 - Search for =compile_commands.json= in build sub-directory
   (thanks to Amos Bird and Eivind Fonn)

layers/+tools/xclipboard/README.org

@@ -8,6 +8,16 @@
 * Description
 =xclipboard= integration layer.
 
+** Requirements
+
+This layer depends on a few platform-specific command-line tools:
+
+- on OSX, this layer calls =pbcopy=
+- on Windows, this layer calls =clip.exe=
+- on GNU/Linux systems, this layer relies on =xsel= to be available.
+
+Note that =xsel= might not be installed by default on e.g. Ubuntu systems.
+
 ** Features:
 - adds copy support to the X-clipboard from the terminal.
 - adds paste support to the X-clipboard from the terminal.