add links to book chapters

cpu-api/README.md

@@ -1,6 +1,6 @@
 ## CPU API
 
-This code is basically that found in [Interlude: Process API](http://pages.cs.wisc.edu/~remzi/OSTEP/cpu-api.pdf).
+Code from OSTEP chapter [Interlude: Process API](http://pages.cs.wisc.edu/~remzi/OSTEP/cpu-api.pdf).
 
 To compile, just type:
 ```

intro/README.md

@@ -1,8 +1,13 @@
 # Overview
 
-Type "make" to build programs.
+Code from OSTEP chapter [Introduction](http://pages.cs.wisc.edu/~remzi/OSTEP/intro.pdf).
 
-See primitive "Makefile" for details.
+To compile, just type:
+```
+prompt> make
+```
+
+See the highly primitive `Makefile` for details.
 
 Then, run them! Examples: