add user documentation

README.md

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+## Configuration
+
+The timer display and settings are entirely configured on a per-player basis via configuration formspecs. Configurations are set when leaving the formspec via the buttons or when pressing enter or clicking the apply button (where available). Pressing the escape key cancels the changes.
+
+For opening the main menu simply enter the `/mtimer` chat command. From the main menu all configuration formspecs are accessible.
+
+![Main Menu](screenshots/main_menu.png)
+
+Simply click the button of the option you want to configure.
+
+### Variables
+
+Some formspecs allow you to use variables. The variables and their current (i.e. the moment when opening the formspec) values are shown below the input field.
+
+![Real-World Time Format](screenshots/real_world_time_format.png)
+
+Simply use the variables as shown. For example, to use the day name in the formspec shown above simply enter `{dname}`. Using unknown variables is possible. Those variables are rendered literal. So Entering `{foobar}` renders to “{foobar}” in the output.
+
+Using a string being a variable is not possible (`{foobar}` will always render to it’s value if it is a valid variable). Using the same variable multiple times is possible.
+
+### Confirmations
+
+While pressing enter in an input field applies the changes not all formspecs have input fields. Sometimes there only schemes or just buttons.
+
+Using those formspecs is the same except setting things works instantly. So by clicking the bottom right on the scheme in the formspec below directly places the timer there.
+
+![Position](screenshots/position.png)
+
+Where needed, apply buttons are placed. So instead of having to leave the formspec via the navigation buttons at the bottom to apply the changes simply click the apply button to have the value applied.
+
+![Timer Format](screenshots/timer_format.png)
+
+In the timer format configuration formspec shown above a text area is used where pressing enter goes to a new line so a button for applying the changes is shown.
+
+## Known Issues
+
+### Formspecs Look Weird
+
+On very small screens the configuration formspecs might look very crammed with text being cut off or overlaying other text. This is due to how formspecs work and is not fixable without massive engine changes.
+
+Same in the other direction: On very large screens the formspecs seem unnecessary huge with much empty space. Same here: This is not fixable without re-implementing formspec generation from scratch in Minetest.
+
+mTimer was tested with resolutions from 1024×768 up to 2560×1440 with 20px font and GUI scaling of 1.5. Since different font sizes and GUI scaling are somewhat broken in Minetest when it comes to formspecs the later was usable but did not look that good.
+
+### Formspec Language Does Not Match Game Language
+
+Simple: The mod was not translated to the language you’re using. Feel free to translate it and file a pull request. I’d be happy to merge it.
+
+Currently only German (`language = de` in `minetest.conf`) is supported as translation language and English being the default.