[Documentation] Add README.md entry for tensor_decoder

This add the new README.md for tensor_decoder.

As partial solution of #490, this follows the format of other #490 related commits

Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>

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+# NNStreamer::tensor\_decoder
+
+## Supported features
+
+- Direct conversion of other/tensor with video/x-raw semantics back to video/x-raw stream.
+
+## Planned features
+
+- With given properties from users or pipeline developers, support the following conversions. The list is not exclusive and we may need to implement more features later.
+
+
+| Main property (input tensor semantics) | Additional & mandatory property | Output |
+| -------------------------------------- | ------------------- | ------ |
+| Bounding boxes (other/tensor)          | N/A                 | video/x-raw |
+| Image label (other/tensor)             | File path to labels | text/x-raw |
+| Bounding boxes + labels (other/tensors)| File path to labels | video/x-raw (textoverlay processed?) |
+| ... more features coming ... | | |
+
+
+## Sink Pads
+
+One always sink pad.
+
+- other/tensor (current)
+- Either other/tensor or other/tensors (future, planned)
+
+## Source Pads
+
+One always source pad.
+
+- video/x-raw (current)
+- Either video/x-raw or text/x-raw (future, planned)
+
+TBD: we may need to support multiple source pads (bounding boxes + labels?)
+
+## Performance Characteristics
+
+TBD.
+
+## Properties
+
+- output-type: Integer denoting VIDEO, AUDIO, or TEXT.
+  - Experimental: we need to update this. Current "output-type" is not satisfactory.
+
+The following properties are suggested and planned.
+- (update) output-type: String denoting "bounding-boxes", "image-label", "bounding-boxes-with-label", ...
+- additional-file-1: String denoting the file path to the **first** data file for decoding; e.g., label list text file for image labeling.
+- additional-file-2: **second** data file if the corresponding output-type requires two or more.
+- additional-file-N: ... **N'th** data file if the corresponding output-type requires N or more.
+
+
+## Properties for debugging
+
+- silent: disable or enable debugging messages
+
+## Usage Examples
+
+```
+$ gst-launch somevideosrc_with_xraw ! tee name=t ! queue ! tensor_converter ! tensor_filter SOME_OPTION ! tensor_decoder output-type=image-label additional-file-1=/tmp/labels.txt ! txt. t. ! queue ! textoverlay name=txt ! autovideosink
+```
+Note: not tested. not sure if the syntax is correct with ```txt. t. !```. Regard the above as pseudo code.