doc: add a `CONTRIBUTING.md` file
doc: fix typos in `CONTRIBUTING.md`
lib: fix UI bug in explorer (#88)
lib: make the engine less concurrent

This is a “temporary” fix for the issue #89. It makes Ketrew significantly
slower but at least it does not overflow picky SSH servers with many
connections.

The long-term solution should come from #73 and #69.
Merge PR #86 (`CONTRIBUTING.md`)
Merge PR #91 (UI bug in explorer)
Merge PR #93 (quick fix for #89)