Document minimum version for node 12 (closes #665)

Author: vweevers

README.md

@@ -37,7 +37,7 @@ See the section on <a href="#safety">safety</a> below for details of known unsaf
 
 ## Supported Platforms
 
-We aim to support _at least_ Active LTS and Current Node.js releases, Electron 4.0.0, as well as any future Node.js and Electron releases thanks to [N-API](https://nodejs.org/api/n-api.html). Because N-API has an experimental status in node 6 and early 8.x releases, the minimum node version for `leveldown` is `8.6.0`.
+We aim to support _at least_ Active LTS and Current Node.js releases, Electron 4.0.0, as well as any future Node.js and Electron releases thanks to [N-API](https://nodejs.org/api/n-api.html). The minimum node version for `leveldown` is `8.6.0`. Conversely, for node >= 12, the minimum `leveldown` version is `5.0.0`.
 
 The `leveldown` npm package ships with prebuilt binaries for popular 64-bit platforms as well as ARM, Android and Alpine (musl) and is known to work on: