Redfish cheatsheet: Add warning about assuming ids

The Redfish spec discusses how a Redfish client should not assume an id.
This document hardcodes ids like ManagerId and ComputerSystemId.
Add appropriate warning.

Tested:
https://github.com/gtmills/docs/blob/fix-up-redfish-cheatsheet/REDFISH-cheatsheet.md

Change-Id: Ib0c8e0f09c98374757ac9377f1d90147cc682849
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>
diff --git a/REDFISH-cheatsheet.md b/REDFISH-cheatsheet.md
index e2d0b4a..189a1b9 100644
--- a/REDFISH-cheatsheet.md
+++ b/REDFISH-cheatsheet.md
@@ -1,6 +1,11 @@
 # Redfish cheat sheet
-This document is intended to provide a set of [Redfish][1] client commands for OpenBMC usage.
-(Using CURL commands)
+This document is intended to provide a set of example [Redfish][1] client
+commands for OpenBMC usage. This document uses cURL.
+This document assumes several ids, such as ManagerId, "bmc", and
+ComputerSystemId, "system". Assuming an id is not correct and any software
+written to use the Redfish API should not. From the  Redfish Specification,
+DSP0266, "Clients shall not make assumptions about the URIs for the members of a
+resource collection."
 
 ## Query Redfish Service Root
 ```