splitting README.md into files in /docs for better readability

Signed-off-by: Claudius <opensource@amenthes.de>

docs/dev/getting-started.md

@@ -0,0 +1,16 @@
+# Developer Notes
+
+## Structure
+
+```text
+codimd/
+├── test/           --- test suite
+├── docs/           --- documentation
+├── lib/            --- server libraries
+└── public/         --- client files
+    ├── css/        --- css styles
+    ├── docs/       --- default documents
+    ├── js/         --- js scripts
+    ├── vendor/     --- vendor includes
+    └── views/      --- view templates
+```