Add an exemple to retrieve chain info (polkadot-js#125)

* add chain info

* sidebar

Author: Tbaut

docs/api/examples/promise/chain-info.md

@@ -0,0 +1,23 @@
+---
+title: Get chain information
+---
+
+This example shows how to connect to the api and retrieve the chain information such as the token name, the ss58 format for address encoding and the token decimals.
+
+```javascript
+// Import the API
+const { ApiPromise } = require('@polkadot/api');
+
+async function main () {
+  // Create a new instance of the api
+  const api = await ApiPromise.create();
+  // get the chain information
+  const chainInfo = await api.registry.getChainProperties()
+
+  console.log(chainInfo);
+  // for Polkadot this would print
+  // {ss58Format: 0, tokenDecimals: [10], tokenSymbol: [DOT]}
+}
+
+main().catch(console.error).finally(() => process.exit());;
+```

sidebars.js

@@ -44,7 +44,8 @@ module.exports = {
           'api/examples/promise/system-events',
           'api/examples/promise/transfer-events',
           'api/examples/promise/upgrade-chain',
-          'api/examples/promise/typegen'
+          'api/examples/promise/typegen',
+          'api/examples/promise/chain-info',
         ]
       },
       'api/FAQ'