Add doc for how to contribute missing ops. (#4023)

CONTRIBUTING_MISSING_OP.md

@@ -0,0 +1,75 @@
+## How to Contribute a New op to TF.js Repository
+
+### Step 1. Add the new op to tfjs-core ops directory.
+**Prerequisite**
+1. The op should already be supported in [TF](https://www.tensorflow.org/api_docs/python/tf/all_symbols) api.
+2. The op should not have been supported in TF.js, see [op support list](https://github.com/tensorflow/tfjs/blob/master/tfjs-converter/docs/supported_ops.md).
+
+**Implementation Details**
+1. Create a new op in `tfjs-core/ops` directory.
+
+    The op file should have the following elements:
+        1. License information.
+        2. JSDoc comment.
+        3. Input validations, if any.
+        4. Delegate the execution to the right kernel through `ENGINE.runKernel()`.
+
+    In addition, for the kernel delegation to work properly, in `kernel_names.ts`
+  file, define:
+        1. kernel name.
+        2. input type.
+        3. attribute type.
+
+2. Export the op file in ops.ts in the same directory.
+3. Add tests for the op in the same directory. Test file name should be the same as the op file’s name with _test suffix.
+4. Exclude the test in all the backends, and add annotation “Not implemented yet.”. See below for where to exclude the test in each backend:
+
+**cpu backend**
+
+In `run_tests.ts`, `customInclude` method, add:
+```
+    // Not implemented yet.
+    if (testName.includes(test_name)) {
+      return false;
+    }
+```
+
+**webgl backend**
+
+In `setup_test.ts`, `customInclude` method, add:
+```
+    // Not implemented yet.
+    if (testName.includes(test_name)) {
+      return false;
+    }
+```
+
+**node backend**
+
+In `run_tests.ts`, `IGNORE_LIST`, add test_name to the list.
+
+### Step 2. Add a new kernel in a backend.
+**Implementation Details**
+1. Create a new kernel in the `kernels` directory of a backend.
+
+    The kernel file should have the following elements:
+    1. License information.
+    2. The kernel implementation, and export it.
+    3. Export a `kernelConfig`.
+
+2. Register the kernel in `register_all_kernels.ts` in the corresponding backend.
+
+3. Remove the op from test exclusion list in the corresponding backend. For wasm
+   backend, add the test to the inclusion list.
+
+### Step 3. Add the op to Converter’s executor.
+1. Add op mapping in the op list ts file, use your best judgement to assign an op category: `tfjs-converter/src/operations/op_list/{corresponding_op_category}.ts`. Use
+the [TF C++ op api](https://www.tensorflow.org/api_docs/cc/) as reference for tfOpName, inputs, and attrs.
+
+2. Auto generates corresponding json file by running following command in directory tfjs-converter: `yarn gen-json`. Check that the json object is generated in `python/tensorflowjs/op_list/{corresponding_op_group}.json`
+
+3. Find the corresponding executor for the op and add the op to the switch, the [executors](https://github.com/tensorflow/tfjs/tree/master/tfjs-converter/src/operations/executors) are in `tfjs-converter/src/operations/executors`.
+
+4. Add a test to the corresponding executor test file.
+
+5. Update the supported op doc in `tfjs-converter/docs/supported_ops.md`.