Add documentation

[nuest]

Agreement about documentation location: Markdown files within the repository. Advantage: versioned, close to the code, text-based. Also suitable because iceland is for developers only.

Documentation tasks

• for desctructable und constructable, i.e. the iceland lifecycle
• for best practices (or: options) regarding configuration files, see Context loading from module-specific files SOS#242 (comment)
• How to develop an OGC service with iceland?
• Fix all // TODO statements in the code, see https://github.com/52North/iceland/search?utf8=%E2%9C%93&q=%2F%2F+TODO (empty class comments could just be removed...)

[nuest]

@bpross-52n Feed free to add documentation tasks from your perspective to the first entry.

[autermann]

Migrated to 52North/arctic-sea#5.