add readme

README

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+This is a lightweight C++ option parser library, supporting the standard GNU
+style syntax for options.
+
+Options can be given as:
+
+    --long
+    --long=argument
+    --long argument
+    -a
+    -ab
+    -abc argument
+
+where c takes an argument, but a and b do not.
+
+How to use:
+
+    #include <cxxopts.hpp>
+
+Create a cxxopts::Options instance.
+
+    cxxopts::Options options;
+
+Then use add_options.
+
+    options.add_options()
+      ("d,debug", "Enable debugging")
+      ("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.
+
+To parse the command line do:
+
+    options.parse(argc, argv);
+
+To retrieve an option use options.count("option") to get the number of times
+it appeared, and
+
+    options["opt"].as<type>()
+
+to get its value. If "opt" doesn't exist, or isn't of the right type, then an
+exception will be thrown.
+
+
+== TODO list ==
+
+* Remove parsed options.
+* Allow unrecognised options.
+* Multiple occurences of options store in vector.