Updates README (#537)

The original text suggested using `--all-targets` for building.  This
replaces that with `--all-features` as the suggestion for both building
and testing.  Removing `--all-targets`, especially for tests, is good
because doc tests don't get built/run with that parameter.

Adds a clean/build everything script to make it easier to run most of what
CI does.  Adds this to the README as well.

README.md

@@ -30,13 +30,21 @@ $ git submodule update --init --recursive
 Building the project:
 
 ```bash
-$ cargo build --workspace --all-targets
+$ cargo build --workspace --all-features
 ```
 
 Running all tests for `ion-rust`:
 
 ```bash
-$ cargo test --workspace
+$ cargo test --workspace --all-features
+```
+
+Our continuous integration builds/tests, checks formatting, builds all API docs including private,
+and clippy linting, you will likely want to check most of these to avoid having to wait until the
+CI finds it:
+
+```bash
+$ ./clean-rebuild.sh
 ```
 
 [spec]: https://amazon-ion.github.io/ion-docs/docs/spec.html

clean-rebuild.sh

@@ -0,0 +1,23 @@
+#!/bin/bash
+
+set -e
+
+# update the index
+cargo update
+
+# default build
+cargo clean
+cargo test --workspace
+
+# all features
+cargo clean
+cargo test --workspace --all-features
+
+# make sure formatting is good
+cargo fmt -- --check
+
+# make sure we deal with lints
+cargo clippy --workspace --all-features --tests -- -Dwarnings
+
+# general **all** rustdoc
+cargo doc --document-private-items --all-features