add gflags dependency doc

diff --git a/docs/installation.md b/docs/installation.md
index c32f2d0..6ff1887 100644 (file)
--- a/docs/installation.md
+++ b/docs/installation.md
@@ -71,12 +71,22 @@ And on **CentOS or RHEL**, you can install via yum using:
 
     sudo yum install protobuf-devel leveldb-devel snappy-devel opencv-devel boost-devel hdf5-devel
 
-The only exception being the google logging library, which does not exist in the Ubuntu 12.04 or CentOS/RHEL repositories. To install it, do:
+The only exceptions being the google logging library and the google flags library, which does not exist in the Ubuntu 12.04 or CentOS/RHEL repositories. To install them, do:
 
+    # glog
     wget https://google-glog.googlecode.com/files/glog-0.3.3.tar.gz
     tar zxvf glog-0.3.3.tar.gz
     ./configure
     make && make install
+    # gflags
+    wget https://github.com/schuhschuh/gflags/archive/master.zip
+    unzip master.zip
+    cd gflags-master
+    mkdir build && cd build
+    cmake ..
+    make && make install
+
+Note that if you have newer versions of Ubuntu, like 13.04 or 14.04, gflags may be available under apt-get.
 
 #### OS X