Change README to markdown.

exportclight/README → exportclight/README.md

-                        The clightgen tool
-                        ------------------
+# The clightgen tool
 
-OVERVIEW
 
+## Overview
 "clightgen" is an experimental tool that transforms C source files
 into Clight abstract syntax, pretty-printed in Coq format in .v files.
 These generated .v files can be loaded in a Coq session for
 interactive verification, typically.
 
 
-HOW TO BUILD
+## How to build
 
 Change to the top-level CompCert directory and issue
-
-          make clightgen
-
-
-USAGE
-
-        clightgen [options] <C source files>
-
+```
+   make clightgen
+```
+
+## Usage
+```
+   clightgen [options] <C source files>
+```
 For each source file "src.c", its Clight abstract syntax is generated
 in "src.v".
 
 The options recognized are a subset of those of the CompCert compiler ccomp
-(see http://compcert.inria.fr/man/manual003.html for full documentation):
-
+(see [user's manual](http://compcert.inria.fr/man/manual003.html) for full documentation):
+```
    -I<dir>     search <dir> for include files
    -D<symbol>  define preprocessor macro
    -U<symbol>  undefine preprocessor macro
    -Wp,<opts>  pass options to C preprocessor
    -f<feature> activate emulation of the given C feature
-
+```