INSTALL.Linux: Document use of "sparse" with Open vSwitch.

Signed-off-by: Ben Pfaff <blp@nicira.com>

INSTALL.Linux

@@ -100,6 +100,14 @@ you have the following:
 
     - Python 2.x, for x >= 4.
 
+If you are going to extensively modify Open vSwitch, please consider
+installing the following to obtain better warnings:
+
+    - "sparse" version 0.4.4 or later
+      (http://www.kernel.org/pub/software/devel/sparse/dist/).
+
+    - GNU make.
+
 Installation Requirements
 -------------------------
 
@@ -187,6 +195,9 @@ Prerequisites section, follow the procedure below to build.
 
       % make
 
+   For improved warnings if you installed "sparse" (see
+   "Prerequisites"), add C=1 to the "make" command line.
+
 4. Become root by running "su" or another program.
 
 5. Run "make install" to install the executables and manpages into the