README: Add 'Getting started' directions

README.md

@@ -2,3 +2,65 @@ 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](https://mesonbuild.com/Getting-meson.html). 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](https://mesonbuild.com/Build-options.html#build-options) 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-test` and `portal-test-qt`
+
+Set the `build-portal-test` or `build-portal-test-qt` option to `true` and build the project to compile the corresponding binary. Then run the binary, e.g.:
+
+```
+./_build/portal-test/portal-test
+```
+
+or
+
+```
+./_build/portal-test/portal-test-qt
+```