Update readme with API versions (#308)

Author: GabrielAnca

README.md

@@ -51,8 +51,11 @@ $client = new IntercomClient('<insert_token_here>', null, ['Intercom-Version' =>
 
 For more information about API Versioning, please check the [API Versioning Documentation](https://developers.intercom.com/building-apps/docs/api-versioning) and the [API changelog](https://developers.intercom.com/building-apps/docs/api-changelog).
 
+**Important**: Not all the resources supported by this API are supported by all API versions. See the notes below or the [API Reference](https://developers.intercom.com/intercom-api-reference/reference) for more information about the resources supported by each API version.
+
 ## Contacts
-Warning: This resource is only available on version 2.0 of the Intercom API.
+
+This resource is only available in API Versions 2.0 and above
 
 ```php
 /** Create a contact */
@@ -91,6 +94,8 @@ $client->contacts->getContacts([]);
 
 ## Users
 
+This resource is only available in API Versions 1.0 to 1.4. Newer versions use the [Contacts](#contacts) resource instead.
+
 ```php
 /** Create a user */
 $client->users->create([
@@ -158,6 +163,8 @@ See [here](https://github.com/intercom/intercom-php#scroll) for more info on usi
 
 ## Leads
 
+This resource is only available in API Versions 1.0 to 1.4. Newer versions use the [Contacts](#contacts) resource instead.
+
 ```php
 /**
  * Create a lead
@@ -309,15 +316,14 @@ $client->companies->create([
 ]);
 
 /**
- * Update a company (Note: This method is an alias to the create method.
- * In practice you can use create to update companies if you wish)
+ * Update a company
  */
 $client->companies->update([
     "name" => "foocorp",
     "id" => "3"
 ]);
 
-/** Creating or Update a company with custom attributes. */
+/** Create or update a company with custom attributes. */
 $client->companies->update([
     "name" => "foocorp",
     "id" => "3",
@@ -410,7 +416,7 @@ $client->conversations->search([
     "pagination" => ["per_page" => 10]
 ]);
 
-/** Get next page of conversation search results (API version > 2.0) */
+/** Get next page of conversation search results (API version >= 2.0) */
 $client->conversations->nextSearch($query, $response->pages);
 
 /**
@@ -515,7 +521,7 @@ You can grab the next page of results using the client:
 $client->nextPage($response->pages);
 ```
 
-In API version 2.0 subsequent pages for listing contacts can be retreived with:
+In API versions 2.0 and above subsequent pages for listing contacts can be retreived with:
 
 ```php
 $client->nextCursor($response->pages);

src/IntercomClient.php

@@ -21,6 +21,8 @@
 
 class IntercomClient
 {
+    const SDK_VERSION = '4.4.0';
+
     /**
      * @var HttpClient $httpClient
      */
@@ -328,7 +330,8 @@ private function getRequestHeaders()
         return array_merge(
             [
                 'Accept' => 'application/json',
-                'Content-Type' => 'application/json'
+                'Content-Type' => 'application/json',
+                'User-Agent' => 'Intercom-PHP/' . self::SDK_VERSION,
             ],
             $this->extraRequestHeaders
         );