docs: add markdown for development (#138)

Author: rainerhahnekamp

DEVELOPMENT.md

@@ -0,0 +1,18 @@
+Every new feature needs to come with following things:
+
+An RFC on GitHub where it is decided if the feature is going to be implemented or not, including some basic implementation and design details.
+
+SignalStore features start with an `with`, followed by the feature name. For example, `with-encryption`. You create it the source file in `libs/ngrx-toolkit/src/lib`.
+
+If the feature does not fit into one file, divide it up into multiple files and put them into a folder with the same name as the feature. For example, as it is done with `withDevtools()`.
+
+In case the feature uses third-party libraries, we need to provide a secondary entry point. An existing example is the `redux-connector` in `libs/ngrx-toolkit/redux-connector`.
+
+Further necessary things for a new feature:
+
+- Test
+  - Unit Tests
+  - E2E Tests
+- Documentation in
+  - `/docs`
+  - as well at the function itself via JSDoc