[libchamplain] Add more meson-related details into README.md

[Jiří Techet <jiritechet src gnome org>]

commit 74b46446e191df07ca76616a5c4d35223a5fe459
Author: Jiří Techet <techet gmail com>
Date:   Sun Feb 10 18:24:39 2019 +0100

    Add more meson-related details into README.md

 README.md | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)
---
diff --git a/README.md b/README.md
index 4a7d6e7..22f15a6 100644
--- a/README.md
+++ b/README.md
@@ -15,11 +15,17 @@ libchamplain depends on the following libraries:
 * [cairo](https://www.cairographics.org)
 * [sqlite](https://www.sqlite.org)
 
-To build libchamplain from sources using meson, run:
+To build libchamplain from sources using [meson](https://mesonbuild.com), run:
 ```
 meson _builddir; cd _builddir; ninja; sudo ninja install
 ```
 
+It is possible to specify compilation options defined in `meson_options.txt`
+by using the `-D` flag, e.g.:
+```
+meson _builddir -Dgtk_doc=true -Ddemos=true
+```
+
 The **repository and bug report** page is at:
 * https://gitlab.gnome.org/GNOME/libchamplain