Add nlnet as a sponsor (#108)

NLnet is sponsoring Gitea development by funding developers to work on specific features. The application and the allocation of this funding was [discussed on the Gitea forum](https://discourse.gitea.io/t/nlnet-grant-application-for-federation-deadline-october-1st-2021/4073) from the start.

Co-authored-by: Loïc Dachary <loic@dachary.org>
Reviewed-on: https://gitea.com/gitea/theme/pulls/108
Reviewed-by: 6543 <6543@obermui.de>
Reviewed-by: Gusted <williamzijl7@hotmail.com>
Co-authored-by: dachary <dachary@noreply.gitea.io>
Co-committed-by: dachary <dachary@noreply.gitea.io>

layouts/home/index.html

@@ -70,6 +70,11 @@
 					<a href="https://hostea.org/blog/"><img src="/images/hostea.png" style="height:100px" alt="Hostea"></a>
 				</div>
 			</div>
+			<div class="columns" style="align-items: center;justify-content: center;margin-top:15px;">
+				<div class="column">
+					<a href="https://nlnet.nl/project/Gitea/"><img src="/images/nlnet.png" style="height:100px" alt="NLnet"></a>
+				</div>
+			</div>
 		</div>
 	</div>
 </section>