docs: add more information regarding --use-new-run (#2687)

README.md

@@ -1068,8 +1068,12 @@ Set this flag to indicate which build stage is the target build stage.
 
 #### Flag `--use-new-run`
 
-Use the experimental run implementation for detecting changes without requiring
-file system snapshots. In some cases, this may improve build performance by 75%.
+Use the experimental RUN implementation for detecting changes without requiring
+Using this flag enables an experimental implementation of the Run command which does not rely on snapshotting at all.
+In this approach, in order to compute which files were changed, a marker file is created before executing the Run command.
+Then the entire filesystem is walked (takes ~1-3 seconds for 700Kfiles) to find all files whose ModTime is greater than the marker file.
+With this new run command implementation, the total build time is reduced seeing performance improvements in the range of ~75%. This new run mode trades 
+off accuracy/correctness in some cases (potential for missed files in "snapshot") for improved performance by avoiding the full filesystem snapshots.
 
 #### Flag `--verbosity`