Clarify OS requirements

PR to clarify requirements based on discussion in github/ghes#4511

Author: davidjarzebowski

docs/requirements.md

@@ -5,8 +5,7 @@ storage and must have network connectivity with the GitHub Enterprise Server app
 
 ## Backup host requirements
 
-Backup host software requirements are modest: Linux or other modern Unix operating
-system (Ubuntu is highly recommended) with [bash][1], [git][2], [OpenSSH][3] 5.6 or newer, [rsync][4] v2.6.4 or newer, and [jq][11] v1.5 or newer.
+Backup host software requirements are modest: Linux operating (Ubuntu or Debian) system with [bash][1], [git][2], [OpenSSH][3] 5.6 or newer, [rsync][4] v2.6.4 or newer, and [jq][11] v1.5 or newer.
 
 The parallel backup and restore feature will require [GNU awk][10] and [moreutils][9] to be installed.