Add ADR on building our own UI library (#756)

decisions/0002-build-own-ui-library.md

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+# 2. Build our own UI library
+
+Status: Accepted (retrospective)
+
+## Context
+
+Before React, the team behind React UI worked at VisionApps, a digital agency.
+For its client projects it built and used Bootstrap UI, a custom extension of
+Bootstrap.
+
+When the agency moved to React, it decided to build its own UI library in React
+instead of adopting an existing one. That is where the name React UI comes from.
+
+The agency closed in mid 2018. The library stayed, and its developers continued
+working on it as individuals in cooperation with RACOM. React UI has been used in
+only a few projects since then. RACOM is by far the largest of them and is the
+main reason the library still exists.
+
+RACOM's applications have two requirements that ready-made libraries did not
+meet: full control over the components, and a library that is as lightweight as
+possible.
+
+## Decision
+
+We develop and maintain our own UI library rather than building on a
+third-party one.
+
+## Consequences
+
+- We decide the API, the design and what goes in. Nothing changes because an
+  upstream project changed direction.
+- We keep the library small. Size and dependencies are weighed on every change.
+- We carry the full cost of maintenance, accessibility, browser support and
+  documentation ourselves.
+- The generic name is hard to search for and says little about the library.
+  We have kept it.