docs: add DEPRECATION.md

DEPRECATION.md

@@ -0,0 +1,37 @@
+# Deprecation Policy
+
+This software library is meant to evolve rapidly and, as such, does not follow [Qiskit's deprecation policy](https://github.com/Qiskit/qiskit/blob/main/DEPRECATION.md). Nonetheless, we will always try to give users ample time to adjust to changes without breaking code that they have already written.
+
+To avoid dependency issues, exact version specification is encouraged if no upcoming updates are needed (e.g. [version pinning](https://www.easypost.com/dependency-pinning-guide)).
+
+
+## Supported API and pre-releases
+
+Following [Python's naming conventions](https://realpython.com/python-double-underscore/) as outlined by [PEP8](https://peps.python.org/pep-0008/), any module, variable, function, or class whose name starts with a leading underscore `_` will be considered _internal_ and not part of this library's supported _application programming interface_ (API).
+
+Some capabilities may be pre-released before reaching a stable state. These will not adhere to the deprecation policies in place and will actively warn users of their unstable, pre-release, condition until they are deemed otherwise.
+
+Every other piece of source code conforms the _public-facing_ API of this library and will therefore be subject to the rules outlined in this document.
+
+
+## Migrations
+
+In order to avoid redundancy with [Qiskit](https://www.ibm.com/quantum/qiskit) and other IBM software products, once a specific capability from this library gets integrated into IBM's stable product stack we will proceed to its deprecation here.
+
+Said deprecation process will last for at least three months, and will not begin until a _migration guide_ explaining users how to transition to the new product feature is produced, approved, and published.
+
+
+## Deprecations, breaking changes, and versioning
+
+This library follows [semantic versioning](https://semver.org/) (i.e. `MAJOR.MINOR.PATCH`).
+
+In most cases, functionality will not be changed or removed without displaying active warnings for a sufficiently long period of time. During this period, we will keep old interfaces and mark them as _deprecated_. Deprecations, changes and removals are considered API modifications, and can only occur in _minor_ releases, not _patch_ releases.
+
+We may occasionally introduce breaking changes (i.e. backwards incompatible) in order to bring new functionality to users more rapidly, simplify existing tooling, or facilitate maintenance. These changes will only be included in _major_ releases.
+
+Major version zero (i.e. `0.X.Y`) may include breaking changes in minor releases.
+
+
+## Documenting changes
+
+Each substantial improvement, breaking change, or deprecation occurring for each release will be documented in `CHANGELOG.md`.

FILEMAP.md

@@ -8,6 +8,8 @@
   participation in the repository is subject to these conduct guidelines.
 - [`CONTRIBUTING.md`](CONTRIBUTING.md) --
   guidelines to contribute to the repository.
+- [`DEPRECATION.md`](DEPRECATION.md) --
+  deprecation policy.
 - [`FILEMAP.md`](FILEMAP.md) --
   a summary of the repository structure and explanations of the different files and folders.
 - [`INSTALL.md`](INSTALL.md) --