Note abi3-py* features in the guide and Add CHANGELOG

Author: kngwyu

CHANGELOG.md

@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ### Added
 - Add support for building for CPython limited API. This required a few minor changes to runtime behaviour of of pyo3 `#[pyclass]` types. See the migration guide for full details. [#1152](https://github.com/PyO3/pyo3/pull/1152)
+  - Relatedly, `abi3-py*` feature flags are added. [#1263]((https://github.com/PyO3/pyo3/pull/1263))
 - Add argument names to `TypeError` messages generated by pymethod wrappers. [#1212](https://github.com/PyO3/pyo3/pull/1212)
 - Add `PyEval_SetProfile` and `PyEval_SetTrace` to FFI. [#1255](https://github.com/PyO3/pyo3/pull/1255)
 - Add context.h functions (`PyContext_New`, etc) to FFI. [#1259](https://github.com/PyO3/pyo3/pull/1259)

guide/src/building_and_distribution.md

@@ -58,6 +58,14 @@ pyo3 = { version = "...", features = ["abi3"]}
 
 3. Ensure that the `.whl` is correctly marked as `abi3`. For projects using `setuptools`, this is accomplished by passing `--py-limited-api=cp3x` (where `x` is the minimum Python version supported by the wheel, e.g. `--py-limited-api=cp35` for Python 3.5) to `setup.py bdist_wheel`.
 
+### Minimum Python version for `abi3`
+
+We provide `abi3-py36`/`abi3-py37`/... features to set the minimum required Python version for abi3 wheel.
+E.g., if you set `abi3-py36` feature, you can build `cp36-abi3-manylinux2020_x86_64.whl` using Python 3.8.
+
+To ensure ABI compatibility, we don't allow setting a minimum version higher than the system Python version.
+E.g., if you set `abi3-py38` and try to compile the crate with Python 3.6, it just fails.
+
 ## Cross Compiling
 
 Cross compiling PyO3 modules is relatively straightforward and requires a few pieces of software: