Ubuntu install libraw added

INSTALL.md

@@ -6,7 +6,17 @@
 
 2. LibRaw
    1. MacOS : `brew install libraw libpng libjpeg`
-   2. Ubuntu : TODO
+   2. Ubuntu : 
+      step 1: download libraw package here: https://www.libraw.org/download
+      step 2: extract the pkg using `tar xzvf LibRaw-X.YY.tar.gz`
+      step 3: cd LibRaw-X.YY
+               ./configure # with optional args
+               make
+      step 4: sudo make install
+      step 5: change the target_link_libraries in cmakelist file to the following:
+      target_link_libraries(${PROJECT_NAME} 
+              ${OpenCV_LIBS}
+              ${LIBRAW_LIBRARY})
 
 3. Run CMake to build
    ```shell