linking to PyQGIS cookbook for python plugin dev

my suggestion towards #89
Merge pull request #218 from saberraz/master

Change image tag for Lutra
Add header identifier color for documentation section
added sourcepole as sponsor
fixed background of sponsors
Merge pull request #219 from anitagraser/master

created sponsors.png with sourcepole
Merge pull request #212 from anitagraser/patch-1

linking to PyQGIS cookbook for python plugin dev
better links
Merge remote-tracking branch 'refs/remotes/origin/master'
Donor Thomas Leahy added
Donor Kai Kotzian added
changed to new sponsors.png
Donor Mercedes Rubi Jimenez added
Donor Tam Nguyen Van added
Merge pull request #220 from anitagraser/patch-2

changed to new sponsors.png
Donor Alejandro Schwedhelm added
Donor Sarah Wakeford added
Donor Nicholas Bedworth added
Donor Louis Maniere added
Donor Thomas Vieira added