Make it more clear the mtime issue is theoretical

Although we were able to reproduce this with the previous behaviour of
the COPY and ADD commands, we have fixed that issue and our attempts to
cause the issue to occur with RUN did not succeed, so it may be that in
practice this will never happen.

README.md

@@ -365,8 +365,12 @@ When taking a snapshot, kaniko's hashing algorithms include (or in the case of
 if the file has changed. Unfortunately there is a delay between when changes to a
 file are made and when the `mtime` is updated. This means:
 
-* With the default snapshot mode (`--snapshotMode=full`), whether or not kaniko will
-  add a layer in the case where a `RUN` command modifies a file but the contents do
-  not change is non-deterministic.
 * With the time-only snapshot mode (`--snapshotMode=time`), kaniko may miss changes
-  introduced by `RUN` commands entirely.
+  introduced by `RUN` commands entirely.
+* With the default snapshot mode (`--snapshotMode=full`), whether or not kaniko will
+  add a layer in the case where a `RUN` command modifies a file **but the contents do
+  not** change is theoretically non-deterministic. This _does not affect the contents_
+  which will still be correct, but it does affect the number of layers.
+
+_Note that these issues are currently theoretical only. If you see this issue occur, please
+[open an issue](https://github.com/GoogleContainerTools/kaniko/issues)._