Add libevent-dev to README.

Bug: 26906928
Change-Id: Ia4ca1b77c342b9e9a2d21daebb75659a7ff3fa48
Reviewed-on: https://weave-review.googlesource.com/2430
Reviewed-by: Jacob Marble <jacobmarble@google.com>

README.md

diff --git a/README.md b/README.md
index a0dc2e7..211033c 100644
--- a/README.md
+++ b/README.md
@@ -49,7 +49,20 @@
 
 ```
 sudo apt-get update
-sudo apt-get install autoconf automake binutils g++ hostapd libavahi-client-dev libcurl4-openssl-dev libexpat1-dev libnl-3-dev libnl-route-3-dev libssl-dev libtool
+sudo apt-get install \
+  autoconf \
+  automake \
+  binutils \
+  g++ \
+  hostapd \
+  libavahi-client-dev \
+  libcurl4-openssl-dev \
+  libevent-dev \
+  libexpat1-dev \
+  libnl-3-dev \
+  libnl-route-3-dev \
+  libssl-dev \
+  libtool
 ```
 
 # Prerequisites
@@ -72,7 +85,8 @@
   - hostapd
   - libavahi-client-dev
   - libcurl4-openssl-dev
-  - libevent 2.1.x-alpha (included; see third_party/get_libevent.sh)
+  - libevent 2.0.x
+  - libevhtp (included; see third_party/get_libevhtp.sh)
 
 
 # Compiling