Add readme.txt into distribution with usefull information about Windup

Windup distribution contains https://raw.githubusercontent.com/windup/windup-distribution/master/src/main/resources/README.txt which doesn't contain usually useful information about Windup like it is in https://raw.githubusercontent.com/windup/windup-distribution/master/README.asciidoc.

We probably should switch/rename them in Windup distribution.