Update readme.md

readme.md

 # physics
 
 Licensed under GNU GPL 3.0
+
+This is a simple physics simulation. There is a red dot and a bunch of green dots.
+The green dots accelerate towards the red dot as if by gravitational attraction.
+They can collide with eachother, with the walls, and with the red dot.
+You can move the red dot by dragging your cursor over the window.
+
+A few months ago I wrote various SDL2-based programs, but they are very badly
+written (look in my repository "djf-3d" to see what I mean). They are an embarassment.
+I wrote this program to demonstrate that I know now how to do things the Right Way,
+in pure C. C is what I am good at; whenever I write C++, I am really just writing C
+with classes and iostreams. When I have attained true mastery of C, I will pick up C++
+again, this time being sure to use the various features, data structures, and library
+abstractions (vectors, smart pointers, RAII, etc.) that characterize modern C++. Since
+I have not, in my own estimation, attained true mastery of C, I will not be writing C++
+again for a good while.
+
+You will need libsdl2-dev to make this program run. I have tested it on GNU/Linux and
+MacOS X, and it seems to run fine on both platforms without the need to modify the source
+code or the make script. However, my attempts to build and run it on Windows, with MinGW
+or with Visual Studio, have failed. I therefore recommend that you run this on a Unix-like
+operating system. Additionally, the make script is written in bash and relies on gcc.