Add Leshan as Californium adopter

Created by: sbernard31

Following the discussion in https://github.com/EclipseFdn/iot.eclipse.org/pull/402#issuecomment-618012340

Before to merge this PR, It should be decided if it's possible to mark an Eclipse projects as an adopter of another project.

data/adopters.yml

@@ -252,6 +252,11 @@ adopters:
logo: logo-swir.svg
logo_white: logo-swir-white.svg
+    - name: Leshan
+      homepage_url: https://www.eclipse.org/leshan/
+      logo: logo-leshan.svg
+      logo_white: logo-leshan-white.svg
+
- name: Bosch.IO
homepage_url: https://www.bosch-iot-suite.com/service/hub/
logo: logo-bosch.svg
@@ -264,3 +269,4 @@ adopters:
homepage_url: https://www.sierrawireless.com/
logo: logo-swir.svg
logo_white: logo-swir-white.svg
+