update readme file

- add status badge for appveyor CI
- add some external hyperlinks
- other minor edits

README.md

 [![Build Status](https://travis-ci.org/stevengj/nlopt.svg?branch=master)](https://travis-ci.org/stevengj/nlopt)
+[![Build Status](https://ci.appveyor.com/api/projects/status/github/stevengj/nlopt?branch=master&svg=true)](https://ci.appveyor.com/project/StevenGJohnson/nlopt)
 
 NLopt is a library for nonlinear local and global optimization, for
 functions with and without gradient information.  It is designed as
-as simple, unified interface and packaging of several free/open-source
+a simple, unified interface and packaging of several free/open-source
 nonlinear optimization libraries.
 
 The latest release and a complete manual may be found at the NLopt
 home page: http://ab-initio.mit.edu/nlopt
 
-It is compiled and installed with CMake build system:
-
-    cmake .
-    sudo make install
-
-See `CMakeLists.txt` file for cmake options.
-To build the latest development sources:
+NLopt is compiled and installed with the [CMake][1] build system
+(see `CMakeLists.txt` file for available options):
 
     git clone git://github.com/stevengj/nlopt
     cd nlopt
     cmake .
     make
+    sudo make install
 
-(To build from git, you will need SWIG.)
+(To build the latest development sources from git, you will need [SWIG][2]
+to generate the Python and Guile bindings.)
 
 Once it is installed, `#include <nlopt.h>` in your C/C++ programs and
-link it with `-lnlopt -lm`.  You may need to use the C++ compiler to link
+link it with `-lnlopt -lm`.  You may need to use a C++ compiler to link
 in order to include the C++ libraries (which are used internally by NLopt,
-even though it has a C API).
+even though it exports a C API).
 
-The minimization function, `nlopt_minimize`, is described in the [man
-page](http://en.wikipedia.org/wiki/Man_page) (`api/nlopt_minimize.3`,
-which is installed by `make install`.  See also the manual on our
-web page.
+The minimization function, `nlopt_minimize`, is described in the [manpage][3]
+(`api/nlopt_minimize.3`, which is installed by `make install`).
+See also the manual on our web page.
 
-There are also interfaces for Fortran, Python, Matlab, Octave, OCaml,
+There are also interfaces for Fortran, Python, MATLAB, GNU Octave, OCaml,
 GNU Guile, GNU R, Lua, and Julia.  Interfaces for other languages may
 be added in the future.
+
+[1]: https://cmake.org/
+[2]: http://www.swig.org/
+[3]: https://en.wikipedia.org/wiki/Man_pagey