Add README.

README.md

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+# randopix
+## Usage
+```
+randopix
+
+Version 0.1 - Display random images from different sources
+
+Usage:
+  randopix [options] 
+
+Options:
+  -m, --mode=MODE            foxes, inspiro, inspiro-xmas
+  -w, --windowed             Do not start in fullscreen mode
+  -h, --help                 Show this help
+```
+## Build
+Install the [Nim Compoiler](https://nim-lang.org/install.html).
+
+Use this command to install the dependencies and build the program:
+```
+$ nimble release
+```