Add a screenshot and a README

c/sdl/README.md

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# hello\_sdl
While reading [Things Unlearned](https://scattered-thoughts.net/writing/things-unlearned)
by Jamie Brandon recently, I happened upon this paragraph:
> I definitely feel I've gotten better at this. When I wanted to write a
> text editor last year I spent a few days learning the absolute basics
> of graphics programming and text rendering, used mostly mainstream
> tools like sdl and freetype, and then just sat down and shoveled
> through a long todo list. In the end it only took 100 hours or so,
> much less time than I spent thrashing on that note-taking app a decade
> ago. And now I get to use my text editor all the time.
And then I proceeded to render some text using SDL + FreeType.  It took
2 or 3 hours, never having used SDL _or_ FreeType.  That includes
debugging various string-related segfaults in C.  (Don't write to
statically allocated `char *msg = "...";` and don't use `strcmp` with
NULL pointers.)
It was fun!  And hence, this little thing.  You can even type something
yourself.
This is how it looks after those initial few hours:
![screenshot of hello\_sdl running](./screenshot.png)
And for everything else, see [`hello_sdl.c`](./hello_sdl.c).  To run it
run `make` and then run `./hello_sdl`.  Do not that you'll need a TTF
font from somewhere and then run it like this:
```
./hello_sdl /usr/share/fonts/TTF/DejaVuSerif.ttf
```
By default it wants `./FantasqueSansMono-Regular.ttf`, which you can get
from the `ttf-fantasque-sans-mono` package on ArchLinux.