Merge pull request #4016 from mysterywolf/armcreadme

[libc]  add readmes

components/libc/compilers/armlibc/README.md

@@ -0,0 +1,9 @@
+# ARMLIB (Keil-MDK) porting for RT-Thread
+
+Please define RT_USING_LIBC and compile RT-Thread with Keil-MDK compiler.
+
+
+
+## More Information
+
+https://www.keil.com/support/man/docs/armlib/

components/libc/compilers/common/readme.md

@@ -1,4 +1,4 @@
-# Attention : 
+## Attentions
 
 1. This folder is "common" for armlibc newlibc and dlib. It's not "common" for minilibc.
 

components/libc/compilers/dlib/README.md

@@ -1,4 +1,9 @@
-Dlib(IAR) porting for RT-Thread.
+# DLIB (IAR) porting for RT-Thread
 
 Please define RT_USING_LIBC and compile RT-Thread with IAR compiler.
 
+
+
+## More Information
+
+http://www.iarsys.co.jp/download/LMS2/arm/7502/ewarm7502doc/arm/doc/EWARM_DevelopmentGuide.ENU.pdf    P.130

components/libc/compilers/newlib/README.md

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+# NEWLIB (GCC) porting for RT-Thread
+
+Please define RT_USING_LIBC and compile RT-Thread with GCC compiler.
+
+
+
+## More Information
+
+https://sourceware.org/newlib/libc.html#Reentrancy