[geocode-glib/wip/hadess/api-version] README: Add information about libsoup 3.x API version change

[Bastien Nocera <hadess src gnome org>]

commit b6442029a76b5384b4505d2da4926c4959789c28
Author: Bastien Nocera <hadess hadess net>
Date:   Thu Jun 30 12:05:21 2022 +0200

    README: Add information about libsoup 3.x API version change

 README | 6 ++++++
 1 file changed, 6 insertions(+)
---
diff --git a/README b/README
index afb023d..50bb65d 100644
--- a/README
+++ b/README
@@ -9,6 +9,12 @@ Requirements
 
 geocode-glib requires json-glib, gio and libsoup to compile and run.
 
+While geocode-glib can be compiled against libsoup version 2.x or version 3.x, as
+those versions can't cohabit in the same binary, you will need to choose which
+one your application will support, especially if geocode-glib isn't a direct
+dependency. Version 1.0 of the geocode-glib API only supports libsoup 2.x,
+version 2.0 (and later) of the API will only support libsoup 3.x.
+
 Applications and libraries using geocode-glib
 ---------------------------------------------