Added a readme file to imglab that gives a little introduction

tools/imglab/README.txt

+imglab is a simple graphical tool for annotating images with object bounding
+boxes.  Generally, you use it when you want to train an object detector (e.g. a
+face detector) since it allows you to easily create the needed training
+dataset.   
+
+You can compile imglab with the following commands:
+    cd dclib/tools/imglab
+    mkdir build
+    cd build
+    cmake ..
+    cmake --build . --config Release
+Note that you may need to install CMake (www.cmake.org) for this to work.  On a
+unix system you can also install imglab into /usr/local/bin by running 
+    sudo make install  
+This will make running it more convenient.
+
+Next, to use it, lets assume you have a folder of images called /tmp/images.
+These images should contain examples of the objects you want to learn to
+detect.  You will use the imglab tool to label these objects.  Do this by
+typing the following command:
+    ./imglab -c mydataset.xml /tmp/images
+This will create a file called mydataset.xml which simply lists the images in
+/tmp/images.  To add bounding boxes to the objects you run:
+    ./imglab mydataset.xml
+and a window will appear showing all the images.  You can use the up and down
+arrow keys to cycle though the images and the mouse to label objects.  In
+particular, holding the shift key, left clicking, and dragging the mouse will
+allow you to draw boxes around the objects you wish to detect.  
+
+Once you finish labeling objects go to the file menu, click save, and then
+close the program. This will save the object boxes back to mydataset.xml.  You
+can verify this by opening the tool again with:
+    ./imglab mydataset.xml
+and observing that the boxes are present.
+
+
+imglab can do a few additional things.  To see these run:
+    imglab -h 
+and also read the instructions in the About->Help menu.
+