doc: add v8 fast api contribution guidelines

doc/contributing/adding-v8-fast-api.md

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+# Adding V8 Fast API
+
+Node.js uses [V8](https://github.com/v8/v8) as the JavaScript engine.
+In order to provide fast paths for functions that are called quite often,
+V8 proposes the usage of `fast api calls` that does not use any of the V8
+internals, but uses internal C functions.
+
+## Limitations
+
+* Fast API functions can not use JavaScript heap allocation.
+* Fast API functions can not trigger JavaScript execution.
+* Reporting/returning errors is not available on fast API, but can be done
+  through fallbacks to slow API.
+* Not all parameter and return types are supported in fast API calls.
+  For a full list, please look into
+  [`v8-fast-api-calls.h`](../../deps/v8/include/v8-fast-api-calls.h) file.
+
+## Requirements
+
+* Each unique fast API function signature should be defined inside the
+  [`external references`](../../src/node_external_reference.h) file.
+* To test fast APIs, make sure to run the tests in a loop with a decent
+  iterations count to trigger V8 for optimization and to prefer the fast API
+  over slow one.
+
+## Fallback to slow path
+
+Fast API supports fallback to slow path (implementation that uses V8 internals)
+in case logically it is wise to do so, for example when providing a more
+detailed error. Fallback mechanism can be enabled, and changed from both caller
+JavaScript function or from the fast API function declaration.
+
+Every fast API function accessible from JavaScript side can pass an object
+consisting of fallback key with a boolean value as the last parameter
+(or the first parameter, if no parameters of the function exist).
+
+In V8 the options fallback is defined as `FastApiCallbackOptions` inside
+[`v8-fast-api-calls.h`](../../deps/v8/include/v8-fast-api-calls.h) file.
+
+* JavaScript land
+
+  Example of a JavaScript call into a fast API:
+
+  ```js
+  // Let divide be a function that provides fast api calls.
+  const { divide } = internalBinding('custom_namespace');
+
+  function divide(a, b) {
+    return divide(a, b, { fallback: a === 0 })
+  }
+  ```
+
+* C++ land
+
+  Example of a conditional fast path on C++
+
+  ```cpp
+  // Anywhere in the execution flow, you can set fallback and stop the execution.
+  static double divide(const int32_t a,
+                       const int32_t b,
+                       v8::FastApiCallbackOptions& options) {
+    if (b == 0) {
+      options.fallback = true;
+    } else {
+      return a / b;
+    }
+  }
+  ```
+
+## Example
+
+A typical function that communicates between JavaScript and C++ is as follows.
+
+* On the JavaScript side:
+
+  ```js
+  const { divide } = internalBinding('custom_namespace');
+  ```
+
+* On the C++ side:
+
+  ```cpp
+  #include "v8-fast-api-calls.h"
+
+  namespace node {
+  namespace custom_namespace {
+
+  static void divide(const FunctionCallbackInfo<Value>& args) {
+    Environment* env = Environment::GetCurrent(args);
+    CHECK(args[0]->IsInt32());
+    CHECK(args[1]->IsInt32());
+    auto a = args[0].As<v8::Int32>();
+    auto b = args[1].As<v8::Int32>();
+
+    if (b->Value() == 0) {
+      return node::THROW_ERR_INVALID_STATE(env, "Error");
+    }
+
+    double result = a->Value() / b->Value();
+    args.GetReturnValue().Set(result);
+  }
+
+  static double FastDivide(const int32_t a,
+                           const int32_t b,
+                           v8::FastApiCallbackOptions& options) {
+    if (b == 0) {
+      options.fallback = true;
+    } else {
+      return a / b;
+    }
+  }
+
+  CFunction fast_divide_(CFunction::Make(FastDivide));
+
+  static void Initialize(Local<Object> target,
+                         Local<Value> unused,
+                         Local<Context> context,
+                         void* priv) {
+    SetFastMethod(context, target, "divide", Divide, &fast_divide_);
+  }
+
+  void RegisterExternalReferences(ExternalReferenceRegistry* registry) {
+    registry->Register(Divide);
+    registry->Register(FastDivide);
+    registry->Register(fast_divide_.GetTypeInfo());
+  }
+
+  } // namespace custom_namespace
+  } // namespace node
+
+  NODE_BINDING_CONTEXT_AWARE_INTERNAL(custom_namespace,
+                                      node::custom_namespace::Initialize);
+  NODE_BINDING_EXTERNAL_REFERENCE(
+                        custom_namespace,
+                        node::custom_namespace::RegisterExternalReferences);
+  ```
+
+* Update external references ([`node_external_reference.h`](../../src/node_external_reference.h))
+
+  Since our implementation used
+  `double(const int32_t a, const int32_t b, v8::FastApiCallbackOptions& options)`
+  signature, we need to add it to external references.
+
+  Example declaration:
+
+  ```cpp
+  using CFunctionCallbackReturningDouble = double (*)(const int32_t a,
+                                                      const int32_t b,
+                                                      v8::FastApiCallbackOptions& options);
+  ```