docs: add documentation on substitution and shell variable expansion (#…

…38)

Using double quotes for the REPLACEMENT argument with capture group
substitution in shells like Bash or zsh will not work as intended. For
example, a "${1}" will be replaced by the content of the shell variable 1
before rnr can read them, resulting in no substitution.

README.md

@@ -478,6 +478,7 @@ rnr -f '(\w+)-(\d+).(\w+)' '${2}-${1}.${3}' ./*
 ├── 02-file.txt
 └── 03-file.txt
 ```
+__SHELL NOTE:__ In shells like Bash and zsh, make sure to wrap the `REPLACEMENT` pattern in single quotes. Otherwise, capture group indices will be replaced by expanded shell variables.
 #### Capture several named groups and swap them
 1. Capture two digits as `number`.
 2. Capture extension as `ext`.

src/app.rs

@@ -88,7 +88,7 @@ pub fn create_app<'a>() -> App<'a, 'a> {
         )
         .arg(
             Arg::with_name("REPLACEMENT")
-                .help("Expression replacement")
+                .help("Expression replacement (use single quotes for capture groups)")
                 .required(true)
                 .validator_os(is_valid_string)
                 .index(2),