signal: Add an appveyor build file

It is not possible to test much on windows but this will at least verify
that it can be built

appveyor.yml

@@ -0,0 +1,35 @@
+environment:
+  matrix:
+    # Stable channel
+    - TARGET: x86_64-pc-windows-gnu
+      CHANNEL: stable
+    - TARGET: x86_64-pc-windows-msvc
+      CHANNEL: stable
+    # Beta channel
+    - TARGET: x86_64-pc-windows-msvc
+      CHANNEL: beta
+    # Nightly channel
+    - TARGET: x86_64-pc-windows-msvc
+      CHANNEL: nightly
+
+# Install Rust and Cargo
+# (Based on from https://github.com/rust-lang/libc/blob/master/appveyor.yml)
+install:
+  - curl -sSf -o rustup-init.exe https://win.rustup.rs
+  - rustup-init.exe --default-host %TARGET% --default-toolchain %CHANNEL% -y
+  - set PATH=%PATH%;C:\Users\appveyor\.cargo\bin
+  - rustc -Vv
+  - cargo -V
+
+# 'cargo test' takes care of building for us, so disable Appveyor's build stage. This prevents
+# the "directory does not contain a project or solution file" error.
+# source: https://github.com/starkat99/appveyor-rust/blob/master/appveyor.yml#L113
+build: false
+
+test_script:
+  - cargo build --example ctrl-c
+  - rustdoc --test README.md -L target/debug/deps
+
+branches:
+  only:
+    - master