[gimp-web/testing] Add Liberapay in the Donating page.

[Jehan <jehanp src gnome org>]

commit 17e7e956627f437c738d3689fa43667371f309af
Author: Jehan <jehan girinstud io>
Date:   Thu Apr 18 13:50:34 2019 +0200

    Add Liberapay in the Donating page.
    
    We created an account GIMP on Liberapay but it seems to have been a bit
    abandonned lately. I think nobody knows about it as it is not advertized
    anywhere on official GIMP media, ever since we first announced it on
    social networks.

 content/donating/index.md | 6 ++++++
 1 file changed, 6 insertions(+)
---
diff --git a/content/donating/index.md b/content/donating/index.md
index afc8c32c..c07192bc 100644
--- a/content/donating/index.md
+++ b/content/donating/index.md
@@ -35,6 +35,12 @@ Many thanks to all our [sponsors](sponsors.html)!
 
 [Donate to gimp gnome org]: 
https://www.paypal.com/cgi-bin/webscr?cmd=_donations&business=gimp%40gnome%2eorg&lc=US&item_name=Donation%20to%20GIMP%20Project&item_number=106&currency_code=USD
 
+### Liberapay
+
+[Donate to GIMP on Liberapay][liberapay]. Liberapay is a platform run by a non-profit organization allowing 
monthly crowdfunding (subscription based). See Wikipedia's page on 
[Liberapay](https://en.wikipedia.org/wiki/Liberapay).
+
+[liberapay]: https://liberapay.com/GIMP/
+
 ### Flattr
 
 [Donate to GIMP with Flattr][flattr]. Flattr is a microdonation system. Users are able to pay a small amount 
every month (minimum 2 euros) and then click Flattr buttons on sites to share the money they paid among those 
sites, comparable to an Internet tip jar (for more details see Wikipedia's 
[Flattr](http://en.wikipedia.org/wiki/Flattr) article).