update doc

Author: skywind3000

README.md

@@ -243,7 +243,10 @@ eval "$(lua /path/to/z.lua --init zsh once)"
 source (lua /path/to/z.lua --init fish once | psub)
 ````
 
-It could be much faster on slow hardware or Cygwin/MSYS.
+When `add once` mode is off (default), z.lua will consider the time you spent in the directory (like z.sh), and when this mode is on, z.lua will consider the times you accessed the directory (like autojump).
+
+It could be much faster on slow hardware or Cygwin/MSYS. 
+
 
 ## Interactive Selection
 
@@ -282,7 +285,7 @@ NOTE: For fish shell, this feature requires fish 2.7.0 or above. You can specify
 
 New option `"-b"` can quickly go back to a specific parent directory in bash instead of typing "cd ../../.." redundantly.
 
-- **(No argument)**: `cd` into the project root, the project root is one of the parent directory with `.git`/`.hg`/`.svn` in it
+- **(No argument)**: `cd` into the project root, the project root the nearest parent directory with `.git`/`.hg`/`.svn` in it
 - **(One argument)**: `cd` into the closest parent starting with keyword, if not find, goto the parent containing keyword.
 - **(Two arguments)**: replace the first value with the second one (in the current path).