doc(README): document supported platforms

Author: Stebalien

README.md

@@ -1,5 +1,4 @@
-tempfile
-========
+# tempfile
 
 [![Crate](https://img.shields.io/crates/v/tempfile.svg)](https://crates.io/crates/tempfile)
 [![Build Status](https://github.com/Stebalien/tempfile/actions/workflows/ci.yml/badge.svg?branch=master)](https://github.com/Stebalien/tempfile/actions/workflows/ci.yml?query=branch%3Amaster)
@@ -11,8 +10,7 @@ patterns and surprisingly difficult to implement securely).
 
 [Documentation](https://docs.rs/tempfile/)
 
-Usage
------
+## Usage
 
 Minimum required Rust version: 1.63.0
 
@@ -23,8 +21,27 @@ Add this to your `Cargo.toml`:
 tempfile = "3"
 ```
 
-Example
--------
+## Supported Platforms
+
+This crate supports all major operating systems:
+
+- Linux
+- Android
+- MacOS
+- Windows
+- FreeBSD (likely other BSDs but we don't have CI for them)
+- RedoxOS
+- Wasm (build and link only, Wasm doesn't have a filesystem)
+- WASI P1 & P2.
+
+However:
+
+- Android, RedoxOS, Wasm, and WASI targets all require the latest stable rust compiler.
+- WASI P1/P2 does not define a default temporary directory. You'll need to explicitly call `tempfile::env::override_temp_dir` with a valid directory or temporary file creation will panic on this platform.
+- WASI P1/P2 does not have file permissions.
+- You _may_ need to override the temporary directory in Android as well to point at your application's per-app cache directory.
+
+## Example
 
 ```rust
 use std::fs::File;