update Features.md, add new customized py layers

Author: vera121

FEATURES.md

@@ -11,9 +11,12 @@ pad_l, pad_r, pad_t and pad_b (arbitrary 2D padding) in ConvolutionParameter and
 AVE_EXC_PAD (average pooling excluding the paddings), AVE_TF (deprecated, alias for AVE_EXC_PAD) in PoolingParameter  
 ceil_mode in PoolingParameter  
 relu6 in ReLUParameter  
-pad.py (customized Python layer)  
-stridedslice.py (customized Python layer)  
-shape.py (customized Python layer)  
+pad.py (customized Python layer, implementation of tf.pad)  
+stridedslice.py (deprecated, customized Python layer)  
+slice.py (customized Python layer, implementation of tf.slice and tf.strided_slice)  
+shape.py (customized Python layer, implementation of tf.shape)  
+stack.py (customized Python layer, implementation of tf.stack)  
+eltwise.py (customized Python layer, functional expansion of ELtwise layer)  
 
 evprune (Network Pruning Tool) related  
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