Add minimal CI workflow using GitHub Actions

.github/workflows/ci.yml

@@ -0,0 +1,26 @@
+name: CI
+on: [push, pull_request]
+
+env:
+  RUSTFLAGS: -D warnings
+  RUST_BACKTRACE: 1
+  CARGO_TERM_COLOR: always
+
+jobs:
+  unit-test:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+      - run: cargo test
+  rustfmt:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@nightly
+        with:
+          components: rustfmt
+      - run: cargo fmt -- --check

README.md

@@ -1,6 +1,6 @@
 # encoding_rs
 
-[![Build Status](https://travis-ci.org/hsivonen/encoding_rs.svg?branch=master)](https://travis-ci.org/hsivonen/encoding_rs)
+[![Build Status](https://github.com/hsivonen/encoding_rs/actions/workflows/ci.yml/badge.svg)](https://github.com/hsivonen/encoding_rs/actions/workflows/ci.yml)
 [![crates.io](https://img.shields.io/crates/v/encoding_rs.svg)](https://crates.io/crates/encoding_rs)
 [![docs.rs](https://docs.rs/encoding_rs/badge.svg)](https://docs.rs/encoding_rs/)