Added extra info in help message (about version & enabled features.)

Fortune for rematch's current commit: Half blessing − 半吉

src/main.rs

@@ -116,8 +116,22 @@ fn main() -> eyre::Result<()>
     let args: re::FrozenVec<re::FrozenString> = std::env::args().map(String::into_boxed_str).collect();
 
     if args.len() < 4 {
+	println!("rematch v{}: Regular-expression group matcher", env!("CARGO_PKG_VERSION"));
+	println!("");
 	println!("Usage: {} <str> <regex> <group>...", args[0]);
 	println!("Pass `-' as `<str>' to read lines from stdin");
+	println!("");
+	println!("Enabled Features:");
+	if cfg!(feature="perl") {
+	    println!("+perl\t\t\tEnable PCRE2 (extended) regular-expressions.\n\t\t\tNote that PCRE2 regex engine matches on *bytes*, not *characters*; meaning if a match cuts a vlid UTF8 codepoint into an invalid one, the output will replace the invalid characters with U+FFFD REPLACEMENT CHARACTER.");
+	} else {
+	    println!("-perl\t\tPCRE2 (extended) features are disabled; a faster but less featureful regular expression engine (that matches on UTF8 strings instead of raw bytes) is used instead.");
+	}
+	if cfg!(feature="unstable") {
+	    println!("+unstable\t\tUnstable optimisations evailable & enabled for build.");
+	} else {
+	    println!("-unstable\t\tUnstable optimisations disabled / not available for build.");
+	}
 	std::process::exit(1)
     } else {
 	let re = re::Regex::compile(&args[2])?;