Set up PR webhook

[rkratky]

Please, set up a webhook to CentOS CI for this repo (or give me maintainer rights to this repo, so I could do it). Here are @riuvshin's notes:

Add the Github Pull Request Builder webhook

1. On your Github Project page, choose 'Settings'
2. Navigate to the 'Webhooks and Services' tab
3. Choose 'Add a webhook'
4. Select 'Let me select individual events' under 'Which events would you like to trigger this webhook?'
5. Unselect 'Push' and select 'Pull Request' and 'Issue Comment'
6. Paste 'https://ci.centos.org/ghprbhook/' (note the trailing slash) in the Payload URL
7. Open the new webhook and verify the ping the Recent Deliveries section

Please, reassign to an appropriate person as you see fit.