Describe the Course model in the specifications.

SPECIFICATIONS.md

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+
+## Courses Model
+
+Each Course MUST have:
+
+- one Identifier _(unique, URL-safe, path-safe)_
+- one Title
+- one Content
+- one Topic
+- one Language `ISO 639-1` (two letter code, eg: `fr`)
+
+Each course MAY have
+
+- one or more Courses as dependencies
+- one or more Tags
+
+
+## TODO
+
+- Look up https://schema.org/Course and comply somehow