Create usermods folder (#389)

usermods/readme.md

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+### Usermods
+
+This folder serves as a repository for usermods (custom `wled06_usermod.ino` files)!
+
+If you have created an usermod that you believe is useful (for example to support a particular sensor, display, feature...), feel free to contribute by opening a pull request!
+
+In order for other people to be able to have fun with your usermod, please keep these points in mind:
+
+- Create a folder in this folder with a descriptive name (for example `usermod_ds18b20_temp_sensor_mqtt`)  
+- Include your custom `wled06_usermod.ino` file
+- If your usermod requieres changes to other WLED files, please write a `readme.md` outlining the steps one has to take to use the usermod
+- Create a pull request!
+- If your feature is useful for the majority of WLED users, I will consider adding it to the base code!
+
+While I do my best to not break too much, keep in mind that as WLED is being updated, usermods might break.  
+I am not actively maintaining any usermod in this directory, that is your responsibility as the creator of the usermod.
+
+Thank you for your help :)