Document shader hot reloading

README.md

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 # OpenGL Template
 
 Just a simple OpenGL template that I use on my streams.
+
+## Shaders Hot Reloading
+
+Press <kbd>F5</kbd> to reload [main.frag](./main.frag) and [main.vert](./main.vert) shaders. Red screen indicates a compilation or linking error, check the output of the program if you see it.