Add API and README docs

API.md

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+# API documentation for [`sfinv_buttons`]
+
+Use this API to add your own buttons to the page.
+
+## `sfinv_buttons.register_button(name, def)
+
+* `name`: Unique button identifier
+* `def`: Coniguration table (see below)
+
+### Fields of `def`
+* `title`: Human-readable text on the button
+* `action(player)`: Function is called when button is triggered
+    * `player`: ObjectRef to player who triggered the button
+    * Default: Nothing happens
+* `tooltip: A tooltip for the button (optional, only use it for an additional, non-critical explanation)	
+* `image`: Texture name for the image to show left of the button (optional)
+* `show(player)`: Shall return `true` if the button should be displayed (optional)
+    * `player`: ObjectRef to player who triggered the button
+    * Default: Always shows button
+
+## Limits
+
+* Up to 18 buttons can be registered
+
+## Recommendations
+
+* Try to summarize the most important information on the button
+* Test if the button title still fits when having >= 9 and <= 18 buttons at a resolution of 800×600
+* It is recommended to only add buttons to access core mod functionality and not for every minor feature. The rule of thumb is to add at most 1 button per mod
+* If you like to add many buttons, consider adding your own `sfinv` page instead 

README.md

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+# Simple Fast Inventory Buttons [`sfinv_buttons`]
+This mod adds a new tab (“More”) to the Simple Fast Inventory [`sfinv`] on
+which other mods can add their buttons to make it easier to access their
+core functionality.
+
+The idea is to free users from typing in lame-ass chat commands. ;-)
+
+## API
+The API is documented in `API.md`.
+
+## Mod license
+MIT License