docs(isBemEntityName): add isBemEntityName method

Author: blond

README.md

@@ -105,6 +105,7 @@ API
 * [isEqual(entityName)](#isequalentityname)
 * [toString()](#tostring)
 * [valueOf()](#valueof)
+* [isBemEntityName()](#isbementityname)
 
 ### constructor(obj)
 
@@ -193,7 +194,6 @@ Parameter    | Type            | Description
 -------------|-----------------|-----------------------
 `entityName` | `BemEntityName` | The entity to compare.
 
-
 Determines whether specified entity is the deepEqual entity.
 
 ```js
@@ -233,6 +233,23 @@ name.valueOf();
 // ➜ { block: 'button', mod: { name: 'focused', value: true } }
 ```
 
+### isBemEntityName()
+
+Determines whether specified entity is instance of BemEntityName.
+
+Parameter    | Type            | Description
+-------------|-----------------|-----------------------
+`entityName` | `BemEntityName` | The entity to check.
+
+```js
+const BemEntityName = require('@bem/entity-name');
+
+const entityName = new BemEntityName({ block: 'input' });
+
+BemEntityName.isBemEntityName(entityName); // true
+BemEntityName.isBemEntityName({}); // false
+```
+
 Debuggability
 -------------