[ffigen] Add a warning message to objective C support (#292)

* Add a warning message to objective C support

* Upgrade warning to severe, and add to readme

pkgs/ffigen/README.md

diff --git a/pkgs/ffigen/README.md b/pkgs/ffigen/README.md
index 32137b9..3665b12 100644
--- a/pkgs/ffigen/README.md
+++ b/pkgs/ffigen/README.md
@@ -483,6 +483,20 @@
       'dart-type': 'CustomType3'
 ```
   </td>
+  <tr>
+    <td>language</td>
+    <td>
+      <b>WARNING:</b> Other language support is EXPERIMENTAL. The API may change
+      in a breaking way without notice.
+      <br><br>
+      Choose the input langauge. Must be one of 'c', or 'objc'. Defaults to 'c'.
+    </td>
+    <td>
+
+```yaml
+language: 'objc'
+```
+  </td>
   </tr>
 </tbody>
 </table>

pkgs/ffigen/lib/src/config_provider/spec_utils.dart

diff --git a/pkgs/ffigen/lib/src/config_provider/spec_utils.dart b/pkgs/ffigen/lib/src/config_provider/spec_utils.dart
index aff84b2..9d7263e 100644
--- a/pkgs/ffigen/lib/src/config_provider/spec_utils.dart
+++ b/pkgs/ffigen/lib/src/config_provider/spec_utils.dart
@@ -464,7 +464,12 @@
 
 bool languageValidator(List<String> name, dynamic value) {
   if (value is String) {
-    if (value == strings.langC || value == strings.langObjC) {
+    if (value == strings.langC) {
+      return true;
+    }
+    if (value == strings.langObjC) {
+      _logger.severe('Objective C support is EXPERIMENTAL. The API may change '
+          'in a breaking way without notice.');
       return true;
     }
     _logger.severe("'$name' must be one of the following - "