[calls] readme: Mention that documentation is available as gitlab page

[Evangelos Ribeiro Tzaras <devrtz src gnome org>]

commit 40cb9edf43c31456b91c3cc4d745eca194ffedf5
Author: Evangelos Ribeiro Tzaras <devrtz fortysixandtwo eu>
Date:   Sun Jan 23 20:42:46 2022 +0100

    readme: Mention that documentation is available as gitlab page

 README.md | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/README.md b/README.md
index 6a955714..1f1578e7 100644
--- a/README.md
+++ b/README.md
@@ -33,6 +33,8 @@ with `-Ggtk_doc=true`
     ninja -C _build
     ninja -C _build calls-doc
 
+You can also browse the documentation [online](https://gnome.pages.gitlab.gnome.org/calls/)
+
 ## Running from the source tree
 
 The most comfortable way to run from the source tree is by using the provided