add README to testdata folder

Author: JohannesKaufmann

testdata/README.md

@@ -0,0 +1,17 @@
+## Testdata
+
+_The "testdata" folder holds various combinations of inputs and expected outputs to check that the library still works._
+
+**You found a problematic HTML snippet? Great!**
+
+Add your problematic HTML snippet to one of the `input.html` files in the `testdata` folder. Then run `go test -update` and have a look at which `.golden` output files changed in GIT.
+
+_Note:_ Adding big block of HTML is not that helpful, so please:
+
+1. narrow it down to the basic HTML structure that still causes the problem,
+2. remove any unnecessary attributes (e.g. data-attributes) and
+3. generalise the content (e.g. update links, replace text with lorem ipsum).
+
+**=> Make sure it has been changed enough to be considered your own work!**
+
+You can rerun `go test -update` until you are happy with the test case that you want to commit. Thanks for expanding the test cases!