Add deprecation / API removal policy (#6852)

* Add deprecation / API removal policy

* Increase proposal to 2 releases

* change from policy to guidelines, add flexibility

* prettier

* Make instructions more actionable

Author: alamb

README.md

@@ -83,6 +83,33 @@ versions approximately every 2 months.
 
 [`object_store`]: https://crates.io/crates/object_store
 
+### Deprecation Guidelines
+
+Minor releases may deprecate, but not remove APIs. Deprecating APIs allows
+downstream Rust programs to still compile, but generate compiler warnings. This
+gives downstream crates time to migrate prior to API removal.
+
+To deprecate an API:
+
+- Mark the API as deprecated using `#[deprecated]` and specify the exact arrow-rs version in which it was deprecated
+- Concisely describe the preferred API to help the user transition
+
+The deprecated version is the next version which will be released (please
+consult the list above). To mark the API as deprecated, use the
+`#[deprecated(since = "...", note = "...")]` attribute.
+
+Foe example
+
+```rust
+#[deprecated(since = "51.0.0", note = "Use `date_part` instead")]
+```
+
+In general, deprecated APIs will remain in the codebase for at least two major releases after
+they were deprecated (typically between 6 - 9 months later). For example, an API
+deprecated in `51.3.0` can be removed in `54.0.0` (or later). Deprecated APIs
+may be removed earlier or later than these guidelines at the discretion of the
+maintainers.
+
 ## Related Projects
 
 There are several related crates in different repositories

arrow/README.md

@@ -37,7 +37,7 @@ This crate is tested with the latest stable version of Rust. We do not currently
 
 The `arrow` crate follows the [SemVer standard] defined by Cargo and works well
 within the Rust crate ecosystem. See the [repository README] for more details on
-the release schedule and version.
+the release schedule, version and deprecation policy.
 
 [SemVer standard]: https://doc.rust-lang.org/cargo/reference/semver.html
 [repository README]: https://github.com/apache/arrow-rs

parquet/README.md

@@ -36,7 +36,7 @@ This crate is tested with the latest stable version of Rust. We do not currently
 
 The `parquet` crate follows the [SemVer standard] defined by Cargo and works well
 within the Rust crate ecosystem. See the [repository README] for more details on
-the release schedule and version.
+the release schedule, version and deprecation policy.
 
 [semver standard]: https://doc.rust-lang.org/cargo/reference/semver.html
 [repository readme]: https://github.com/apache/arrow-rs