A BUILDING.md

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+# Build thing
+
+thing uses the [meson](https://mesonbuild.com) build system.
+
+## Current build process
+
+The following is my current build process, it might not work on every other system.
+
+### libthingterm
+
+```sh
+meson setup build
+meson compile -C build
+meson install -C build
+```
+
+### thing
+
+```sh
+PKG_CONFIG_PATH=/usr/local/lib/pkgconfig meson setup build
+meson compile -C build
+# optionally install
+meson install -C build
+```