added section on how to scale display on x11

x11.md

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+# X11
+
+<!--toc:start-->
+- [X11](#x11)
+<!--toc:end-->
+
+
+## Scaling of hdpi displays (high dots per inch)
+
+If working on setups with varying resolutions, configuring e.g. the font
+size of a terminal emulator might be difficult.
+X11 allows to scale the (entire) display via X resources.
+Edit/create the file `~/.Xresources` and add the following line:
+
+```sh
+Xft.dpi: <dpi>
+
+# example:
+Xft.dpi: 118
+```
+
+To apply the changes execute `xrdb -merge ~/.Xresources`.
+
+To make the changes permanent add `xrdb -merge ~/.Xresources` to your
+`xinitrc` file, if not present already.