Add general documentation on weak-node-api

Author: kraenhansen

docs/WEAK-NODE-API.md

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+# The `weak-node-api` library
+
+Android's dynamic linker impose restrictions on the access to global symbols (such as the Node-API free functions): A dynamic library must explicitly declare any dependency bringing symbols it needs as `DT_NEEDED`.
+
+The implementation of Node-API is split between Hermes and our host package and to avoid addons having to explicitly link against either, we've introduced a `weak-node-api` library (published in `react-native-node-api` package). This library expose only Node-API and will have it's implementation injected by the host.
+
+While technically not a requirement on non-Android platforms, we choose to make this the general approach across across React Native platforms. This keeps things aligned across platforms, while exposing just the Node-API without forcing libraries to build with suppression of errors for undefined symbols.