doc: add guide for updating N-API API surface

doc/guides/adding-new-napi-api.md

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+# Contributing a new API to N-API
+
+N-API is Node.js's next generation ABI-stable API for native modules.
+While improving the API surface is encouraged and welcomed, the following are
+a set of principles and guidelines to keep in mind while adding a new
+N-API API.
+
+* A new API **must** adhere to N-API API shape and spirit.
+    * **Must** be a C API.
+    * **Must** not throw exceptions.
+    * **Must** return `napi_status`.
+    * **Should** consume `napi_env`.
+    * **Must** operate only on primitive data types, pointers to primitive
+      datatypes or opaque handles.
+    * **Must** be a necessary API and not a nice to have. Convenience APIs
+      belong in node-addon-api.
+    * **Must** not change the signature of an existing N-API API or break
+      ABI compatibility with other versions of Node.js.
+* New API **should** be agnostic towards the underlying JavaScript VM.
+* New API PRs **must** have a corresponding documentation update.
+* New API PRs **must** be tagged as **n-api**.
+* There **must** be at least one test case showing how to use the API.
+* There **should** be at least one test case per interesting use of the API.
+* There **should** be a sample provided that operates in a realistic way
+  (operating how a real addon would be written).
+* A new API **should** be discussed at the N-API working group meeting.
+* A new API addition **must** be signed off by at least two members of
+  the N-API WG.
+* A new API addition **should** be simultaneously implemented in at least
+  one other VM implementation of Node.js.
+* A new API **must** be considered experimental for at least one minor
+  version release of Node.js before it can be considered for promotion out
+  of experimental.
+    * Experimental APIs **must** be documented as such.
+    * Experimental APIs **must** require an explicit compile-time flag
+      (`#define`) to be set to opt-in.
+    * Experimental APIs **must** be considered for backport.
+    * Experimental status exit criteria **must** involve at least the
+      following:
+        * A new PR **must** be opened in `nodejs/node` to remove experimental
+          status. This PR **must** be tagged as **n-api** and **semver-minor**.
+        * Exiting an API from experimental **must** be signed off by the
+          working group.
+        * If a backport is merited, an API **must** have a down-level
+          implementation.
+        * The API **should** be used by a published real-world module. Use of
+          the API by a real-world published module will contribute favorably
+          to the decision to take an API out of experimental status.
+        * The API **must** be implemented in a Node.js implementation with an
+          alternate VM.