API 5 released

[andrieslouw]

We've just released API version 5, which contains many enhancements and new features. API 5 is the successor to API 4 (#141).

For more information: https://images.weserv.nl/news/2019/09/01/introducing-api-5/
The new source code has been published in the 5.x branch.

API 5 also brings better documentation: https://images.weserv.nl/docs/
Contributions to the documentation can be made to our documentation repository.

The default branch will switch to 5.x within a couple of days, after which the 4.x branch will be retired.

[uok]

Thanks, your great work and continuous improvements are very appreciated!