Fix 2015/yang to compile everything

A rather unfortunate typo in the Makefile made make try and compile
already compiled C++ object code as source code.

Whilst I was at it I format fixed (and fixed other things I saw) the
README.md file.

2015/yang/Makefile

@@ -112,7 +112,7 @@ ENTRY= yang
 PROG= prog
 #
 OBJ= ${PROG}.o
-CSRC= ${PROG}.o
+CSRC= ${PROG}.c
 CPLUSPLUSSRC= ${PROG}.cc
 DATA=
 TARGET= ${PROG} ${PROG}_c90 ${PROG}_c99 ${PROG}_c11 ${CPLUSPLUSSRC} \