README: Document how to override runc version

Let's make this visible, as several users seem to need this.

Signed-off-by: Rodrigo Campos <rodrigoca@microsoft.com>

README.md

@@ -86,6 +86,14 @@ sudo make install
 
 `runc` will be installed to `/usr/local/sbin/runc` on your system.
 
+#### Version String Customize
+
+You can see the runc version by running `runc --version`. You can also override the version that
+will be printed, using the `VERSION` variable for make when you compile, e.g.:
+```bash
+make VERSION="1.1.13-build#1"
+```
+
 #### Build Tags
 
 `runc` supports optional build tags for compiling support of various features,