Add index.md

Author: felixmulder

docs/index.md

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+Dotty Documentation
+===================
+The Dotty compiler is currently somewhat lacking in documentation - PRs
+welcome! But, we've attempted to gather the most essential knowledge in these
+pages.
+
+Index
+-----
+* [Blog](blog/) posts written for scala-lang.org or dotty.epfl.ch
+* [Contributing](contributing/)
+    - [Eclipse](contributing/eclipse.md) setting up dev environment
+    - [Intellij-IDEA](contributing/intellij-idea.md) setting up dev environment
+    - [Getting Started](contributing/gettings-started.md) details on how to run
+      tests, use the cli scripts
+    - [Workflow](contributing/workflow.md) common dev patterns and hints
+* [Internals](internals/) documents the compiler internals
+    - [Backend](internals/backend.md) details on the bytecode backend
+    - [Contexts](internals/contexts.md) details the use of `Context` in the
+      compiler
+    - [Higher Kinded Type Scheme](internals/higher-kinded-v2.md)
+      scheme
+    - [Project Structure](internals/overall-structure.md)
+      of the project
+    - [Periods](internals/periods.md)
+* [Usage](usage/)
+    - [Migrating from Scala 2](usage/migrating.md)
+    - [Using Dotty with sbt](usage/sbt-projects.md)
+* [Differences between DOT and Dotty](difference-dot-dotty.md) differences between the type
+  calculus and Dotty
+* [Dotty vs Scala2](dotc-scalac.md)
+* [Type System](type-system.md)