Add more info to readme (#62)

Author: matrixik

README.md

@@ -1,6 +1,6 @@
 # slug
 
-Package `slug` generate slug from unicode string, URL-friendly slugify with
+Package `slug` generate slug from Unicode string, URL-friendly slugify with
 multiple languages support.
 
 [![Go Reference](https://pkg.go.dev/badge/github.com/gosimple/slug.svg)](https://pkg.go.dev/github.com/gosimple/slug)
@@ -43,10 +43,34 @@ func main() {
 }
 ```
 
-### Requests or bugs?
+## Design
+
+This library will always returns clean output from any Unicode string
+containing only the following ASCII characters:
+
+* numbers: `0-9`
+* small letters: `a-z`
+* big letters: `A-Z` (only if you set `Lowercase` to `false`)
+* minus sign: `-`
+* underscore: `_`
+
+Minus sign and underscore characters will never appear at the beginning or
+the end of the returned string.
+
+Thanks to context-insensitive transliteration of Unicode characters to ASCII
+output returned string is safe for URL slugs and filenames.
+
+## Requests or bugs?
 
 <https://github.com/gosimple/slug/issues>
 
+If your language is missing you could add it in `languages_substitution.go`
+file.
+
+In case of missing proper Unicode characters transliteration to ASCII you could
+add them to underlying library:
+<https://github.com/gosimple/unidecode>.
+
 ## Installation
 
 ```shell