doc: discuss concurrency in README

Author: whereswaldon

README.md

@@ -856,6 +856,9 @@ There has been several attempts to implement case sensitivity, but unfortunately
 
 You can vote for case sensitivity by filling out this feedback form: https://forms.gle/R6faU74qPRPAzchZ9
 
+### Is it safe to concurrently read and write to a viper?
+
+No, you will need to synchronize access to the viper yourself (for example by using the `sync` package). Concurrent reads and writes can cause a panic.
 
 ## Troubleshooting