README: Add python-serial and git to dependency list.

Suggested in https://github.com/pfalcon/esp-open-sdk/pull/57 .

README.md

@@ -40,7 +40,8 @@ tools installed: gcc, binutils, flex, bison, etc.
 ## Ubuntu 14.04
 ```
 $ sudo apt-get install make unrar autoconf automake libtool gcc g++ gperf \
-    flex bison texinfo gawk ncurses-dev libexpat-dev python sed
+    flex bison texinfo gawk ncurses-dev libexpat-dev python python-serial sed \
+    git
 ```
 
 ## MacOS: