Add docs

Author: dplusic

docs/BUILD.md

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+## Build Instruction
+
+1. `npm install`
+1. `npm run build`

src/Google/README.md

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+## How to generate `Sdk.js`, `Sdk.d.ts`
+
+### Prerequisite
+* [protobufjs](https://www.npmjs.com/package/protobufjs)
+
+### Steps
+
+1. Get `sdk.proto` from https://github.com/dplusic/GameLift-Server-Protobuf/releases
+1. `pbjs -t static-module -w commonjs -o Sdk.js --force-long sdk.proto`
+1. `pbts -o Sdk.d.ts Sdk.js`