update readme

README

@@ -1,3 +1,5 @@
+== Quick start ==
+
 This is a lightweight C++ option parser library, supporting the standard GNU
 style syntax for options.
 
@@ -27,7 +29,8 @@ Then use add_options.
       ("f,file", "File name", cxxopts::value<std::string>())
 
 Options can be declared with a short and/or long option. A description must be
-provided. The third argument is the value, if omitted it is boolean.
+provided. The third argument is the value, if omitted it is boolean. Any type
+can be given as long as it can be parsed, with operator>>.
 
 To parse the command line do:
 
@@ -41,9 +44,19 @@ it appeared, and
 to get its value. If "opt" doesn't exist, or isn't of the right type, then an
 exception will be thrown.
 
+== Linking ==
+
+Compile with -lcxxopts.
+
+== Requirements ==
+
+The only build requirement is a C++ compiler that supports C++11 regular 
+expressions. For example GCC >= 4.9 or clang with libc++.
+
 
 == TODO list ==
 
 * Allow unrecognised options.
 * Print help: format description properly
 * Option groups.
+* INSTALL library.