[doc] Add README.md for developers

This patch adds README.md for developers to build and test
nnstreamer-edge-sensor library.

Signed-off-by: Sangjung Woo <sangjung.woo@samsung.com>

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+# nnstreamer-edge-sensor
+
+`nnstreamer-edge-sensor` is a utility library for publishing any type of data as an MQTT message.
+Without GStreamer dependencies, it can provide remote nodes with stream input data such as camera image, temperature, or humidity. 
+`mqttsrc` element of NNStreamer can access these data and exploit them on pipeline stream.
+
+`nnstreamer-edge-sensor` is designed not to depend on GStreamer/GLib so targets to be adopted by general RTOS or Lightweight devices.
+
+
+## Getting Started
+
+### Prerequisites
+
+To build `nnstreamer-edge-sensor` in the Ubuntu/Debian environment, the following packages are needed.
+
+```bash
+$ sudo apt install cmake libpaho-mqtt-dev libpaho-mqtt1.3 mosquitto mosquitto-clients
+```
+
+### How to build
+
+When building `nnstreamer-edge-sensor`, you can set the version number of nnstreamer-edge-sensor or enable test program.
+In this example, the test program is enabled for a future test. (`-DENABLE_TEST=ON`)
+
+```bash
+$ git clone https://github.com/nnstreamer/nnstreamer-edge.git
+$ cd nnstreamer-edge
+$ mkdir build
+$ cd build
+...
+# In this example, test is enabled.
+$ cmake .. -DCMAKE_INSTALL_PREFIX=/usr -DENABLE_TEST=ON
+$ make
+```
+
+Note that, to install the built library, you need the `root` permission. You can make an rpm or debian package and install it.
+
+
+## How to use
+
+`nnstreamer-edge-sensor` provides a devel package and it contains `pkg-config` file. So you can use it when building your own program.
+
+```bash
+$ pkg-config --libs nnstreamer-edge-sensor
+-L -lnnstreamer-edge-sensor
+```
+
+## How to test
+
+In this example, we publish a jpeg image as `TestTopic` name on the client-side. On the server-side, we subscribe to the `TestTopic` and show received images.
+
+### Check the mqttsrc and mqttsink are available
+
+Before executing the server script, you have to check the `mqttsrc` and `mqttsink` are available as below.
+
+```bash
+$ gst-inspect-1.0 --gst-plugin-path="build" | grep mqtt
+mqtt:  mqttsink: MQTT sink
+mqtt:  mqttsrc: MQTT source
+```
+
+If `mqtt` element is not available in your environment, you can build the NNStreamer from the source code after enabling the mqtt-support option.
+
+```bash
+$ git clone https://github.com/nnstreamer/nnstreamer.git
+$ cd nnstreamer
+
+# Enable the mqtt-support option as below
+# meson_options.txt
+# +option('mqtt-support', type: 'feature', value: 'enabled')
+
+$ meson build
+$ ninja -C build
+
+# If you want to install, add `install` option as below
+$ ninja -C build install
+```
+
+### Server side
+
+With `mqttsrc` element, the below command subscribes `TestTopic` message which contains one jpeg image. If the message is received, then it is shown on the screen. In this example, we built the nnstreamer in the `build` directory so we use `--gst-plugin-path` option to set the mqtt plugin path.
+
+```bash
+$ gst-launch-1.0 --gst-plugin-path="build" mqttsrc debug=1 sub-topic=TestTopic host=localhost sub-timeout=9223372036854775807 ! jpegdec ! video/x-raw,framerate=0/1 ! videoscale ! videoconvert ! ximagesink qos=0
+```
+
+### Publish single data
+
+If you want to publish a single jpeg file, you can use `-f [file_path]` option. Then the received image is shown on another window.
+
+```bash
+$ pwd
+[your_home]/nnstreamer-edge/build
+$ ./tests/test_edge_sensor
+Usage: ./tests/test_edge_sensor [-f jpeg_file] [-d directory] [-c count]
+$ ./tests/test_edge_sensor -f ../tests/res/1.jpg
+```
+![Publish single data](../../tests/res/edge_sensor_00.jpg)
+
+
+### Publish multiple data
+If you want to publish multiple jpeg files, you can use `[-d directory] [-c count]` options.
+As soon as the topic is received, the image is shown as below.
+
+```bash
+$ pwd
+[your_home]/nnstreamer-edge/build
+$ ./tests/test_edge_sensor -d ../tests/res -c 10
+```
+![Publish multiple data](../../tests/res/edge_sensor_result.webp)

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