Update README.md

Author: pwolnows

tools/accuracy_checker/accuracy_checker/adapters/README.md

@@ -43,6 +43,7 @@ AccuracyChecker supports following set of adapters:
   * `joining_method` - method used to join embeddings (optional, supported methods are `sum` and `concatenation`, default - `sum`).
   * `target_out` - target output layer name (Optional, if not provided first in the model will be used).
   * `keep_shape` - allow keeping initial shape for predicted embedding (Optional, default `False`, it means that model output will be flattenized).
+  * `mean_pooling` - average the embeddings of all tokens from last_hidden_state (Optional, default `False`)
 * `yolo_v2` - converting output of YOLO v2 family models to `DetectionPrediction` representation.
   * `classes` - number of detection classes (default 20).
   * `anchors` - anchor values provided as comma-separated list or one of precomputed: