Add features documentation

Author: roligugus

book/src/build/cargo.md

@@ -144,6 +144,46 @@ manifest key][links]* in the Cargo reference.
 
 [links]: https://doc.rust-lang.org/cargo/reference/build-scripts.html#the-links-manifest-key
 
+## Features
+
+CXX provides features to enable additional or alternative APIs.
+
+Features are enabled on the CXX crate for Cargo-based setups, and with compiler
+options for non-Cargo setups.
+
+The following features are available, see `[features]` section in [Cargo.toml](Cargo.toml) for a full list:
+<table>
+<tr><th>feature</th><th>compiler option</th><th>description</th></tr>
+<tr><td>c++14</td><td>-std=c++14 (gcc, clang), /std:c++14 (msvc)</td><td>Compile CXX with c++14</td></tr>
+<tr><td>c++17</td><td>-std=c++17 (gcc, clang), /std:c++17 (msvc)</td><td>Compile CXX with c++17.
+Enables string_view APIs for rust::str and rust::String</td></tr>
+</table>
+
+You will have to enable the feature on the CXX crate *as well as* add the
+equivalent compiler option to your cargo build.
+
+Example to e.g. enable C++17:
+```toml,hidelines
+## Cargo.toml
+[dependencies]
+cxx = { version = "0.4", features = ["c++17"] }
+```
+
+```rust,noplayground
+// build.rs
+
+fn main() {
+    cxx_build::bridge("src/main.rs")  // returns a cc::Build
+        .file("src/demo.cc")
+        .flag_if_supported("-std=c++17")
+        .compile("cxxbridge-demo");
+
+    println!("cargo:rerun-if-changed=src/main.rs");
+    println!("cargo:rerun-if-changed=src/demo.cc");
+    println!("cargo:rerun-if-changed=include/demo.h");
+}
+```
+
 <br><br><br>
 
 # Advanced features