Add documentatoin to document the --preamble argument

Signed-off-by: Ramzi El-khater <ramzie@meta.com>

README.md

@@ -627,6 +627,14 @@ You can mark all fakes with the weak attribute like so:
 
 See the example project that demonstrates the above approach: _./examples/weak_linking_.
 
+## Specifying a preamble in the mock funtion name
+
+You can make fff add a preamble to the mock function names by passing the argument --preamble <string> to fakegen.rb when generating fff.h. This is useful if one needs to build with the real function included for whatever reason. Note that references to the real function will need to change to refer to the mock function.
+
+For example, we can name the mock of function foo() as __wrap_foo() and use the GNU linker option -Wl,--wrap foo to re-route all references to foo() to __wrap_foo() (see https://ftp.gnu.org/old-gnu/Manuals/ld-2.9.1/html_node/ld_3.html). Now, inside __wrap_foo() we can invoke foo() in addition to instrumentation. We can leverage the 'foo_fake.custom_fake' field to call the real version of foo(), this will now be named as
+__real_foo()
+
+
 ## Find Out More
 
 Look under the examples directory for full length examples in both C and C++.