[DOC] Mention git --exec-path in README.

Author: rentzsch

README.md

@@ -6,7 +6,7 @@ git-subtree allows subprojects to be included within a sub-directory of a main p
 
 Choose **one** of the following ways to install git-subtree:
 
-1. Copy the file `git-subtree.sh` to where all other git scripts are stored.
+1. Copy the file `git-subtree.sh` to where all other git scripts are stored (`git --exec-path` will tell you this).
 1. Run `install.sh` in a Git-enabled shell (that's "Git Bash" on Windows).
 1. Run `make install` in a Cygwin-enabled shell.