README.md: update the error scenario output

The pldmtool errors out when it receives a wrong response
for the request sent. The README is updated with the
output of the error scenario.

Change-Id: I832de9a11bb92a964a24bdc2d7cb38b60d7f9ca2
Signed-off-by: Pavithra Barithaya <pavithra.b@ibm.com>
diff --git a/pldmtool/README.md b/pldmtool/README.md
index ae5c611..afc4c58 100644
--- a/pldmtool/README.md
+++ b/pldmtool/README.md
@@ -193,6 +193,24 @@
 ]
 ```
 
+## Understanding pldmtool output error scenario
+
+When the pldmtool receives the wrong response for the request sent it errors out
+with a response code and completion code. The completion code represents the
+type of error and is defined in every pldm type `pldm_<type>_completion_codes`
+enum values.
+
+Example:
+
+This is a platform command and the completion code can be understood from
+[`libpldm/platform.h`](https://github.com/openbmc/libpldm/blob/a98814fc37a5d59207163cb2fa81f4162eaf69cd/include/libpldm/platform.h#L204)
+file.
+
+```
+$ pldmtool platform getpdr -d 17
+Response Message Error: rc=0 , cc=130
+```
+
 ## pldmtool with mctp_eid option
 
 Use **-m** or **--mctp_eid** option to send pldm request message to remote mctp