update document

Author: GorgonMeducer

.gitignore

@@ -21,3 +21,4 @@ RTE_Components.h
 *uvguix.*
 ArInp.bat
 Pre_Include_Global.h
+*.db

README.md

@@ -1,4 +1,4 @@
-# perf_counter (v2.4.0)
+# perf_counter (v2.4.1-dev)
 A dedicated performance counter for Cortex-M Systick. It shares the SysTick with users' original SysTick function(s) without interfering with it. This library will bring new functionalities, such as performance counter,` delay_us` and `clock()` service defined in `time.h`.
 
 ### Features:
@@ -370,7 +370,7 @@ git clone https://github.com/GorgonMeducer/perf_counter.git
 ```
 
 2. Add including path for `perf_counter` folder
-3. Add `perf_counter.c` to your compilation. 
+3. Add `perf_counter.c` and `perfc_port_default.c` to your project for compilation. 
 
 > **NOTE**: Please do **NOT** add any assembly source files of this `perf_counter` library to your compilation, i.e. `systick_wrapper_gcc.S`, `systick_wrapper_gnu.s` or `systick_wrapper_ual.s`.