Making the REAMDME.md a bit more precise and general to use for other people

[marvi3]

I like your approach to make the whole project with just one script.
However I was facing some difficulties when running it and after fixing them I thought I could contribute to the repo b creating a pull request with them.

I changed the following:

• fixed the location of the custom-mutator-helpers according to a newer version of AFL++
• added all steps neccesary for building the project to the README.md
• Added some styling to the README.de
• Added a gitignore
• replaced your path to AFLplusplus in the build.sh by a placeholder

More changes I had no time to fix for now:

• It would be awesome to adapt the repo to the newest AFLplusplus where there is no custom-mutator-helpers.h which is replaced by the afl-fuzz.h. This should not be too much work and if you like I can create a new pull-request sometime in January or February.