update README

README.md

@@ -6,6 +6,38 @@
 
 Written in pure C without librairies.
 
+## Install
+
+You only need **gcc** and **make**.
+
+```bash
+git clone https://github.com/klemek/margen
+cd margen
+make clean build install
+# margen is now installed in ~/.local/bin
+```
+
+## CLI arguments
+
+```txt
+usage: margen [--help] [-q] [-w=WIDTH] [-h=HEIGHT] [-o=PATH] [--seed=SEED][-p=PIXEL_SIZE] [-s=SLOPE] [-c=R,G,B] [-v=R,G,B] [-m]
+
+generates a marble-like pattern bitmap image.
+
+options:
+  --help             show this help message and exit
+  -q, --quiet        do not print to console
+  -w, --width        image width (default: 1920)
+  -h, --height       image height (default: 1080)
+  -o, --output       output file (default: output.bmp)
+  -seed              random seed (default: time based)
+  -p, --pixel        pixel size (default: random)
+  -s, --slope        slope [0-255] (default: random)
+  -c, --color        base color [0-255,0-255,0-255] (default: random)
+  -v, --variation    base variation [0-255,0-255,0-255] (default: random)
+  -m, --monochrome   black & white generation
+```
+
 ## TODO
 
 - monochrome