add examples for setting follow and URL rules; update README with usage instructions

Author: aafeher

README.md

@@ -82,6 +82,48 @@ s = s.SetMultiThread(false)
 s := sitemap.New().SetMultiThread(false)
 ```
 
+#### Follow rules
+
+To set the follow rules, use the `SetFollow()` function. It should be specified a `[]string` value.
+It is a list of regular expressions. When parsing a sitemap index, only sitemaps with a `loc` that matches one of these expressions will be followed and parsed.
+If no follow rules are provided, all sitemaps in the index are followed.
+
+```go
+s := sitemap.New()
+s.SetFollow([]string{
+	`\.xml$`,
+	`\.xml\.gz$`,
+})
+```
+... or ...
+```go
+s := sitemap.New().SetFollow([]string{
+	`\.xml$`,
+	`\.xml\.gz$`,
+})
+```
+
+#### URL rules
+
+To set the URL rules, use the `SetRules()` function. It should be specified a `[]string` value.
+It is a list of regular expressions. Only URLs that match one of these expressions will be included in the final result.
+If no rules are provided, all URLs found are included.
+
+```go
+s := sitemap.New()
+s.SetRules([]string{
+	`product/`,
+	`category/`,
+})
+```
+... or ...
+```go
+s := sitemap.New().SetRules([]string{
+	`product/`,
+	`category/`,
+})
+```
+
 #### Chaining methods
 
 In both cases, the functions return a pointer to the main object of the package, allowing you to chain these setting methods in a fluent interface style:

examples/rules/main.go

@@ -0,0 +1,49 @@
+package main
+
+import (
+	"fmt"
+	"github.com/aafeher/go-sitemap-parser"
+	"log"
+)
+
+func main() {
+	url := "https://versus.com/sitemap_index.xml.gz"
+
+	// create new instance, overwrite default configuration and call Parse() with url
+	s := sitemap.New().SetUserAgent("Mozilla/5.0 (X11; Linux x86_64; rv:123.0) Gecko/20100101 Firefox/123.0").SetFetchTimeout(5).SetMultiThread(false).SetFollow([]string{`/en_phone_162`}).SetRules([]string{"oneplus"})
+	sm, err := s.Parse(url, nil)
+	if err != nil {
+		log.Printf("%v", err)
+	}
+
+	// Print the errors
+	if sm.GetErrorsCount() > 0 {
+		log.Println("parsing has errors:")
+		for i, err := range sm.GetErrors() {
+			log.Printf("%d: %v", i+1, err)
+		}
+	}
+
+	// GetURLCount()
+	count := sm.GetURLCount()
+
+	fmt.Printf("Sitemaps of %s contains %d URLs.\n\n", url, count)
+
+	// GetURLs()
+	urlsAll := sm.GetURLs()
+
+	for i, u := range urlsAll {
+		fmt.Printf("%d. url -> Loc: %s", i, u.Loc)
+		if u.ChangeFreq != nil {
+			fmt.Printf(", ChangeFreq: %v", u.ChangeFreq)
+		}
+		if u.Priority != nil {
+			fmt.Printf(", Priority: %.1f", *u.Priority)
+		}
+		if u.LastMod != nil {
+			fmt.Printf(", LastMod: %s", u.LastMod.String())
+		}
+		fmt.Println()
+	}
+	fmt.Println()
+}