Add Rocket.toml example file; fix syntax

Also ignore a local `Rocket.toml`; it can be used for development
without comitting it.

.gitignore

@@ -1 +1,2 @@
 /target
+Rocket.toml

README.md

@@ -33,11 +33,11 @@ $ cargo run --release
 add `--release`.)
 
 You can provide Rocket with configuration to use a different address and/or port.
-Just create a `Rocket.toml` file that contains:
+Just create a `Rocket.toml` file that contains (or copy `Rocket.toml.example`):
 
 ```toml
 [default]
-address = 0.0.0.0
+address = "0.0.0.0"
 port = 4321
 ```
 

Rocket.toml.example

@@ -0,0 +1,3 @@
+[default]
+address = "0.0.0.0"
+port = 2356