README: Add 'Getting started' directions

README.md

@@ -2,3 +2,76 @@ libportal - Flatpak portal library
 ==================================
 
 libportal provides GIO-style async APIs for most Flatpak portals.
+
+## Getting started
+
+`libportal` uses Meson to build its projects. To install Meson on your system,
+follow the [Getting Meson instructions][1]. If your system is missing a
+dependency, Meson will tell you which one. How you install your missing
+dependencies is up to you.
+
+### Build libportal
+
+The first time you build libportal, give `meson` a directory to build into; we
+recommend `_build`:
+
+```
+meson _build
+```
+
+Then use `ninja` to build the project, pointing it to your build directory:
+
+```
+ninja -C _build
+```
+
+For subsequent builds, you only need to use the `ninja -C _build` command.
+
+### Passing options
+
+libportal includes [Meson build options][2] for components that can optionally
+be built. After first running `meson _build`, you can view the available options
+with:
+
+```
+meson configure _build
+```
+
+To change an option, re-configure the project:
+
+```
+meson configure _build -Dbuild-portal-test=true
+```
+
+You can also pass in options right from the start, e.g. with:
+
+```
+meson _build -Dbuild-portal-test=true
+```
+
+Then build:
+
+```
+ninja -C _build
+```
+
+## Optional components
+
+### `portal-tests`
+
+To generate test binaries, set the `portal-tests` option to any combination of
+`gtk3`, `gtk4`, or `qt` e.g. `-Dportal-tests=gtk3,gtk4,qt`. Then run the desired 
+test binary, e.g.:
+
+```
+./_build/portal-test/portal-test/gtk3/portal-test-gtk3
+```
+
+or
+
+```
+./_build/portal-test/portal-test/qt/portal-test-qt
+```
+
+[1]: https://mesonbuild.com/Getting-meson.html
+[2]: https://mesonbuild.com/Build-options.html#build-options