doc: design for the x package collection (#5)

docs/design/dd-002-x.md

@@ -0,0 +1,23 @@
+# X Design Document
+
+|              |                                                |
+|--------------|------------------------------------------------|
+| Author       | [@bassosimone](https://github.com/bassosimone) |
+| Last-Updated | 2024-11-19                                     |
+
+This document describes the design of the [x](
+https://github.com/rbmk-project/x) library collection,
+which contains several experimental libraries reused across
+[rbmk](https://github.com/rbmk-project) repositories.
+
+The [x](https://github.com/rbmk-project/x) collection and
+repository will never be stable and will never be tagged. It
+is a place for experimentation, and may contained unstable,
+untested, obsolete, or broken code.
+
+When a library added to [x](https://github.com/rbmk-project/x)
+matures, we will copy it either to [common](https://github.com/rbmk-project/common)
+or to a library specific repository. When this happens, we will
+mark the copy of the library living inside the `x` collection
+as deprecated. We will try to keep the old code around, to document
+its history, but we may eventually delete old libraries.