Spell out our platform support policy (#10778)

This reflects how we currently distributed our efforts to supporting the
different platforms and clarifies which things get run by the Nushell
team and which rely on the help of third-parties or individual
contributors.

Feel free to propose improvements, as long as they can be backed up by
implemented practice as a result.

PLATFORM_SUPPORT.md

@@ -0,0 +1,70 @@
+# Nushell platform support policy
+
+Nushell envisions to be a cross-platform shell, despite taking some strong design inspiration from UNIX and POSIX command names and style conventions we explicitly support Windows.
+
+## cross-platform design
+This commitment to a cross-platform Nushell forces us to make provisions so users on Windows can have the generally same pleasant experience: e.g. supporting paths with backslash as the directory separator, forces us to support string literals that accept those.
+
+In general our design strives to have a consistent behavior across all platforms if defining the semantics is possible for Nushell.
+In some cases where the platform requirements dominate we may choose to follow the platform specific defaults. (some nuances around the file system)
+Only rarely do we want to accept commands/language features that only support a single platform, only to access common system behavior of this particular platform (e.g. `registry query` command for the windows registry, `exec` for Linux and MacOS)
+
+## cross-platform builds and testing
+
+The Nushell team runs **testing of Nushell for the following platforms** through our CI:
+
+- macOS (latest version available through GitHub CI)
+- Windows (10 and 11)
+- Linux (our test runners use `ubuntu-20.04` to represent distributions with not the latest glibc versions.)
+
+All PR level tests are performed on x86/AMD64 (at least at the time of writing the default macOS runner was not yet using arm64).
+
+As an additional layer of validation we perform [nightly builds](https://github.com/nushell/nightly/releases).
+
+Those target **additional build targets**:
+- **aarch64 for all platforms**
+- musl as an alternative to Glibc on linux
+- riscv only for linux
+- armv7 only for linux
+
+We will try to provide builds for all of them but a standard configuration for x86-64 or aarch64 will take priority for us should we face technical challenges in a release cycle.
+
+### Supported feature flags
+
+We have features of Nushell behind flags that can be passed at compilation time.
+
+The design focus of Nushell is primarily expressed by everything accessible without passing additional feature flag. This provides a standard command set and receives the most attention.
+Two other feature flags are actively tested but are not guaranteed to express the stable design direction of Nushell:
+- `extra`
+    - This includes commands where we are not convinced that they are ready to be stabilized for 1.0 or popular enough
+- `dataframe`
+    - This includes dataframe support via `polars` and `arrow2`. Introduces a significant additional compilation and binary size.
+    - Due to the use of SIMD extensions may not be compatible with every minimal architecture.
+
+## Passively supported platforms
+
+These platforms are not actively managed through our CI so may encounter unintended regressions.
+Furthermore certain features may not yet be available, even though we are willing to accept PRs trying to close that gap.
+
+
+- OpenBSD
+    - e.g. missing the `ps` command
+- FreeBSD
+    - e.g. missing the `ps` command
+- Android via Termux
+
+Help from the community to make sure they get tested and improved so they can become first class targets would be greatly appreciated!
+
+
+## Providing builds and packaging
+
+The Nushell team only provides a select few distribution sources and so far encourages community members to maintain the individual packages for particular package managers:
+
+We provide:
+- source code distribution via `crates.io` -> `cargo install nu`
+- GitHub builds with each release: (following the build matrix of the nightly builds)
+- the setup for `winget` packaging
+
+### For package maintainers:
+
+We aim to support the rust version that is two releases behind the most recent version of stable Rust so the build infrastructure of your packaging environment can already be proven out.

README.md

@@ -54,6 +54,7 @@ Detailed installation instructions can be found in the [installation chapter of
 
 [![Packaging status](https://repology.org/badge/vertical-allrepos/nushell.svg)](https://repology.org/project/nushell/versions)
 
+For details about which platforms the Nushell team actively supports, see [our platform support policy](PLATFORM_SUPPORT.md).
 
 ## Configuration
 
@@ -198,7 +199,7 @@ topics that have been presented.
 
 Nu adheres closely to a set of goals that make up its design philosophy. As features are added, they are checked against these goals.
 
--   First and foremost, Nu is cross-platform. Commands and techniques should work across platforms and Nu has first-class support for Windows, macOS, and Linux.
+-   First and foremost, Nu is cross-platform. Commands and techniques should work across platforms and Nu has [first-class support for Windows, macOS, and Linux](PLATFORM_SUPPORT.md).
 
 -   Nu ensures compatibility with existing platform-specific executables.