cheatsheet: Add ulimit for coredump

Add `ulimit -c unlimited` to generate coredump, otherwsie the default
coredump limit is 0 and it will not be generated.

Change-Id: I3ca8a4ebc3e9cf6fbf1e45e9077315affb9adaa9
Signed-off-by: Lei YU <mine260309@gmail.com>
diff --git a/cheatsheet.md b/cheatsheet.md
index e0e88a8..5e1295b 100644
--- a/cheatsheet.md
+++ b/cheatsheet.md
@@ -180,6 +180,7 @@
 By default coredump is disabled in OpenBMC. To enable coredump:
 ```
 echo '/tmp/core_%e.%p' | tee /proc/sys/kernel/core_pattern
+ulimit -c unlimited
 ```
 
 ## Cleaning up read-write file system changes