Clarify Readme.

- closes VundleVim#281

Author: gmarik

README.md

@@ -78,10 +78,10 @@
    "filetype plugin on
    "
    " Brief help
-   " :PluginList          - list configured plugins
-   " :PluginInstall(!)    - install (update) plugins
-   " :PluginSearch(!) foo - search (or refresh cache first) for foo
-   " :PluginClean(!)      - confirm (or auto-approve) removal of unused plugins
+   " :PluginList       - lists configured plugins
+   " :PluginInstall    - installs plugins; append `!` to update or just :PluginUpdate
+   " :PluginSearch foo - searches for foo; append `!` to refresh local cache
+   " :PluginClean      - confirms removal of unused plugins; append `!` to auto-approve removal
    "
    " see :h vundle for more details or wiki for FAQ
    " Put your non-Plugin stuff after this line