README: Advertise need of tcpdump and gdb

They are needed by trace.py, which is now run as part of 'make check'
harness.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

README.md

@@ -28,13 +28,13 @@ First, install prerequisite packages:
 **Fedora**
 
 ```
-yum install ant autoconf automake boost-static gcc-c++ genromfs libvirt libtool flex bison qemu-system-x86 qemu-img maven maven-shade-plugin python-dpkt
+yum install ant autoconf automake boost-static gcc-c++ genromfs libvirt libtool flex bison qemu-system-x86 qemu-img maven maven-shade-plugin python-dpkt tcpdump gdb
 ```
 
 **Debian**
 
 ```
-apt-get install build-essential libboost-all-dev genromfs autoconf libtool openjdk-7-jdk ant qemu-utils maven libmaven-shade-plugin-java python-dpkt
+apt-get install build-essential libboost-all-dev genromfs autoconf libtool openjdk-7-jdk ant qemu-utils maven libmaven-shade-plugin-java python-dpkt tcpdump gdb
 ```
 
 **Ubuntu users**: you may use [Oracle JDK][] if you don't want to pull too many