SoftPlus operation specification (#1128)

* Add SoftPlus operation specification

docs/ops/activation/SoftPlus_4.md

@@ -0,0 +1,49 @@
+## SoftPlus <a name="SoftPlus"></a>
+
+**Versioned name**: *SoftPlus-4*
+
+**Category**: *Activation*
+
+**Short description**: SoftPlus takes one input tensor and produces output tensor where the softplus function is applied to the tensor elementwise.
+
+**Detailed description**: For each element from the input tensor calculates corresponding
+element in the output tensor with the following formula:
+
+    \f[
+    SoftPlus(x) = ln(e^{x} + 1.0)
+    \f]
+
+**Attributes**: *SoftPlus* operation has no attributes.
+
+
+**Inputs**:
+
+*   **1**: Multidimensional input tensor of type *T*. **Required**.
+
+**Outputs**:
+
+*   **1**: The resulting tensor of the same shape and type as input tensor.
+
+**Types**
+
+* *T*: arbitrary supported floating point type.
+
+
+**Example**
+
+```xml
+<layer ... type="SoftPlus">
+    <input>
+        <port id="0">
+            <dim>256</dim>
+            <dim>56</dim>
+        </port>
+    </input>
+    <output>
+        <port id="1">
+            <dim>256</dim>
+            <dim>56</dim>
+        </port>
+    </output>
+</layer>
+```

docs/ops/opset4.md

@@ -125,6 +125,7 @@ declared in `namespace opset4`.
 * [Sin](arithmetic/Sin_1.md)
 * [Sinh](arithmetic/Sinh_1.md)
 * [SoftMax](activation/SoftMax_1.md)
+* [SoftPlus](activation/SoftPlus_4.md)
 * [SpaceToBatch](movement/SpaceToBatch_2.md)
 * [SpaceToDepth](movement/SpaceToDepth_1.md)
 * [Split](movement/Split_1.md)