add stability promise

docs/src/SUMMARY.md

@@ -3,7 +3,9 @@
 - [Introduction](./README.md)
   - [Motivation](./background/motivation.md)
   - [Vocabulary](./background/vocab.md)
+  - [Stability Promise](./background/stability-promise.md)
 - [Getting Started](./getting_started.md)
+  - [Sidebar on Operating Systems](./os-sidebar.md)
 - [Tuning the denv](./tune.md)
 - [Command Reference Manual](./manual.md)
   - [denv](./manual/denv.md)

docs/src/background/stability-promise.md

@@ -0,0 +1,20 @@
+# Stability Promise
+
+With the release of version 1.0, denv features a strong
+commitment to backwards compatibility and stability.
+
+Future releases will not introduce backwards incompatible changes
+that make existing denv configurations (mostly represented by the `.denv/config`
+file whose specification is [in the manual](../manual/denv-config.md#files))
+stop working, or break working invocations of the command-line interface.
+
+This does not, however, preclude fixing outright bugs, even if doing so might
+break denv configurations that rely on their behavior.
+
+There will never be a denv 2.0.
+Any desirable backwards-incompatible changes will be opt-in on a per-denv basis,
+so users may migrate at their leisure.
+
+---
+Stability promise text copied heavily from
+[just](https://just.systems/man/en/chapter_9.html).