update doc on how to update dotdrop submodule

README.md

@@ -578,6 +578,7 @@ $ cd <dotfiles-directory>
 
 ## get latest version
 $ git submodule update --init --recursive
+$ git submodule update --remote dotdrop
 
 ## update the bash script wrapper
 $ ./dotdrop/bootstrap.sh

dotdrop/dotdrop.py

@@ -21,6 +21,7 @@ If you want to keep it as a submodule (recommended), simply do the following:
 
 First get the latest version of dotdrop:
     $ git submodule update --init --recursive
+    $ git submodule update --remote dotdrop
 And then re-run the bootstrap script to update \"dotdrop.sh\":
     $ ./dotdrop/bootstrap.sh