WIP: Add canonical example for concepts-basicstrings exercise

[DavyJ0nes]

This is a work in progress to doublecheck that I'm starting along the correct path

Currently this just includes one example but further solutions will be added to explore and add custom suggestions and where the student should be routed, i.e. to a human mentors, more docs etc

The following improvements have also been made so far:

• update module to Go v1.13
• Start on Contributing documentation

[junedev]

The exercise for which this PR adds analyzer comments no longer exists in that form. However the dockerfile update and update of the contributing documentation should be merged. It would be best to create two separate PRs for those.