README: Add 'Getting started' directions

Based on flatpak#29, but a bit expanded and hopefully addressing concerns on that PR. Omits docs instructions for now as I'm not familiar with how that works.

README.md

@@ -2,3 +2,51 @@ libportal - Flatpak portal library
 ==================================
 
 libportal provides GIO-style async APIs for most Flatpak portals.
+
+## Getting started
+
+### The Meson build system
+
+`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.
+
+### Build and run portal-test
+
+If this is your first time building libportal, configure the project to include portal-test:
+
+```
+meson _build -Dbuild-portal-test=true
+```
+
+Otherwise, re-configure the project:
+
+```
+meson configure _build -Dbuild-portal-test=true
+```
+
+Then build:
+
+```
+ninja -C _build
+```
+
+And run the binary:
+
+```
+./_build/portal-test/portal-test
+```