docs(examples): add examples on default values and specific value sets

examples/10_DefaultValues.rs

@@ -0,0 +1,20 @@
+extern crate clap;
+
+use clap::{App, Arg};
+
+fn main() {
+    // You can get a "default value" like feature by using Option<T>'s .unwrap_or() method
+    //
+    // Let's assume you have -c <config> argument to allow users to specify a configuration file
+    // but you also want to support a default file, if none is specified.
+    let matches = App::new("myapp").about("does awesome things")
+                        .arg(Arg::new("CONFIG")
+                                .help("The config file to use (default is \"config.json\")")
+                                .short("c")
+                                .takes_value(true))
+                        .get_matches();
+
+    let config_file = matches.value_of("CONFIG").unwrap_or("config.json");
+
+    // use config_file here...
+}

examples/11_OnlySpecificValues.rs

@@ -0,0 +1,33 @@
+extern crate clap;
+
+use clap::{App, Arg};
+
+fn main() {
+    // If you have arguments of specific values you want to test for, you can use the 
+    // .possible_values() method of Arg
+    //
+    // This allows you specify the valid values for that argument. If the user does not use one of
+    // those specific values, they will receive a graceful exit with error message informing them
+    // of the mistake, and what the possible valid values are
+    //
+    // For this example, assume you want one positional argument of either "fast" or "slow"
+    // i.e. the only possible ways to run the program are "myprog fast" or "myprog slow"
+    let matches = App::new("myapp").about("does awesome things")
+                        .arg(Arg::new("MODE")
+                                .help("What mode to run the program in")
+                                .index(1)
+                                .possible_values(vec!["fast", "slow"])
+                                .required(true))
+                        .get_matches();
+
+    // Note, it's safe to call unwrap() because the arg is required
+    match matches.value_of("MODE").unwrap() {
+        "fast" => {
+            // Do fast things...
+        },
+        "slow" => {
+            // Do slow things...
+        },
+        _      => unreachable!()
+    }
+}