add helloworld to README (rust-lang#134)

* add helloworld to README

Author: miguelraz

README.md

@@ -12,6 +12,40 @@ We can also be found on [Zulip][zulip-project-portable-simd].
 
 If you are interested in support for a specific architecture, you may want [stdarch] instead.
 
+## Hello World
+
+Now we're gonna dip our toes into this world with a small SIMD "Hello, World!" example. Make sure your compiler is up to date and using `nightly`. We can do that by running 
+
+```bash
+rustup update -- nightly
+```
+
+or by setting up `rustup default nightly` or else with `cargo +nihgtly {build,test, run}`. After updating, run 
+```bash
+cargo new hellosimd
+```
+to create a new crate. Edit `hellosimd/Cargo.toml` to be 
+```toml
+[package]
+name = "hellosimd"
+version = "0.1.0"
+edition = "2018"
+[dependencies]
+core_simd = { git = "https://github.com/rust-lang/stdsimd" }
+```
+
+and finally write this in `src/main.rs`:
+```rust
+use core_simd::*;
+fn main() {
+    let a = f32x4::splat(10.0);
+    let b = f32x4::from_array([1.0, 2.0, 3.0, 4.0]);
+    println!("{:?}", a + b);
+}
+```
+
+Explanation: We import all the bindings from the crate with the first line. Then, we construct our SIMD vectors with methods like `splat` or `from_array`. Finally, we can use operators on them like `+` and the appropriate SIMD instructions will be carried out. When we run `cargo run` you should get `[11.0, 12.0, 13.0, 14.0]`.
+
 ## Code Organization
 
 Currently the crate is organized so that each element type is a file, and then the 64-bit, 128-bit, 256-bit, and 512-bit vectors using those types are contained in said file.