doc: Update README.md for user space function tracing

uftrace provides mulitiple ways for user space function tracing, but it
might be confusing to users suggesting all the options in README.md.

We better inform a few options including -pg, -finstrument-functions and
-fpatchable-function-entry=N.

Signed-off-by: Honggyu Kim <[email protected]>

README.md

@@ -28,8 +28,8 @@ the name uftrace stems from the combination of user and ftrace.
 
 It can record data from:
 - User space C/C++/Rust functions (by dynamically patching functions at runtime,
-  or with code compiled with with `-pg`, `-mfentry`, `-finstrument-functions`,
-  or using clang with `-fxray-instrument`)
+  or with code compiled with with `-pg`, `-finstrument-functions` or using
+  `-fpatchable-function-entry=N` for selective NOP patching)
 - C/C++/Rust Library functions (through PLT hooking)
 - Python functions (using Python's trace/profile infrastructure)
 - Kernel functions (using the ftrace framework in Linux kernel)