Update README

Changed:
- Hard-wrapped lines to ~80 characters for consistent reading.
- Updated version information to indicate function is compatible with PHP 7 and PHP 8.
- Improved description of parameters and return values.
- Replaced inline links with reference links.

Author: mcaskill

README.md

@@ -1,6 +1,6 @@
 # html_build_attributes
 
-(PHP 5 >= 5.4)  
+(PHP 5 >= 5.4, PHP 7, PHP 8)  
 `html_build_attributes` — Generate a string of HTML attributes.
 
 ## Description
@@ -13,17 +13,33 @@ Generate a string of HTML attributes from the associative array provided.
 
 ## Parameters
 
-- `attr` — Associative array, or object containing properties, representing attribute names and values.  
-  If `attr` is an object, then only public properties will be incorporated into the result.  
-  If a value is an array, its values will be concatenated and delimited with a space.  
-	Values that cannot be converted into strings will be ignored. If the value is Stringable, Arrayble, or a `Closure`, attempts will be made to parse as a string. 
-- `callback` — Callback function for escaping the values for the HTML attributes.  
-  If no sanitizer is provided, [`htmlspecialchars()`](http://php.net/htmlspecialchars) is used;  
-  If using WordPress, the [`esc_attr()`](https://developer.wordpress.org/reference/functions/esc_attr/) function is used.
+- `attr` — Associative array, or object containing properties, representing
+  attribute names and values.
+  
+  If `attr` is a non-iterable object, then only accessible non-static properties
+  will be incorporated into the result.
+  
+  If a value of `attr` is an array, its values will be concatenated and
+  delimited with a space.
+  
+  If a value of `attr` is a boolean, the value's key will be rendered without
+  a value.
+
+  If the value is Stringable, Arrayble, or a [`Closure`][class.closure],
+  attempts will be made to parse as a string.
+  
+	Values that cannot be converted into strings will be ignored.
+  
+- `callback` — Callback function for escaping the values for the HTML attributes.
+  
+  If no sanitizer is provided, [`htmlspecialchars()`][function.htmlspecialchars]
+  is used;
+  
+  If using WordPress, the [`esc_attr()`][wp.esc_attr] function is used.
 
 ## Return Values
 
-Returns a string of HTML attributes.
+Returns a string of HTML attributes or a empty string if `attr` is invalid or empty.
 
 ## Installation
 
@@ -35,4 +51,10 @@ $ composer require mcaskill/php-html-build-attributes
 
 ### Without Composer
 
-Why are you not using [composer](http://getcomposer.org/)? Download `Function.HTML-Build-Attributes.php` from the gist and save the file into your project path somewhere.
+Why are you not using [composer](http://getcomposer.org/)?
+Download `Function.HTML-Build-Attributes.php` from the gist and save the file
+into your project path somewhere.
+
+[class.closure]:             https://php.net/class.closure
+[function.htmlspecialchars]: https://php.net/function.htmlspecialchars
+[wp.esc_attr]:               https://developer.wordpress.org/reference/functions/esc_attr/