Clarify that the benchmark directory must be empty before the benchmark

README

@@ -36,7 +36,7 @@ where
  - <bytes per thread> is the number of bytes written and read per
    thread (task),
 
- - <directory> is the path to a directory on the filesystem being
+ - <directory> is the path to an empty directory on the filesystem being
    benchmarked.
 
 <threads per node> and <blocksize> will be chosen optimally for best
@@ -72,7 +72,7 @@ where
  - <bytes per thread> is the number of bytes written and read per
    thread (task),
 
- - <directory> is the path to a directory on the filesystem being
+ - <directory> is the path to an empty directory on the filesystem being
    benchmarked.
 
 <threads per node> will be chosen optimally for best performance.
@@ -107,7 +107,7 @@ where
  - <files per thread> is the number of files created, stat, and deleted
    per thread (task),
 
- - <directory> is the path to a directory on the filesystem being
+ - <directory> is the path to an empty directory on the filesystem being
    benchmarked.
 
 <threads per node> will be chosen optimally for best performance.