README: Add description for the tool intended usage

Add description as to how qcom-ptool is internded to be used and list
supported boot flows.

Signed-off-by: Sumit Garg <[email protected]>

Author: b49020

README.md

@@ -1,6 +1,9 @@
 # qcom-tool
 
-qcom-ptool contains various device partitioning utilities like ptool.py, gen_partitions.py and various sample partition configuration files needed for Qualcomm SoCs.
+qcom-ptool contains various device partitioning utilities like ptool.py, gen_partitions.py and various sample partition configuration files needed for Qualcomm SoCs. Qualcomm Linux currently supports two reference Linux based OSes (Yocto with [meta-qcom](https://github.com/qualcomm-linux/meta-qcom) and Debian with [qcom-deb-images](https://github.com/qualcomm-linux/qcom-deb-images)) which uses this tool to generate partition table layouts. The partition GUIDs, names and size budgets are picked to support boot flows as follows:
+
+- (preferred) "edk2/UEFI": PBL => XBL => edk2/UEFI => high-level OS (Linux)
+- (legacy) "U-Boot/UEFI": PBL => XBL => ABL => U-Boot/UEFI => high-level OS (Linux)
 
 # Development