fix SQLiteBlob.md

Author: jepiqueau

docs/SQLiteBlob.md

@@ -4,11 +4,19 @@
 <p align="center">
   Capacitor community plugin for Native and Electron SQLite Databases. For <strong>Native</strong> and <strong>Electron</strong> platforms, databases could be encrypted with SQLCipher</p>
 
+## Prerequisite
+
+```bash
+npm i --save buffer
+```
+
 ## Read a SQLite Blob
 
 ### Image Blob
 
 ```js
+...
+import { Buffer } from 'buffer';
 ...
     const stmt = `SELECT id, name, img FROM testblob;`;
     const ret: DBSQLiteValues = await db.query(stmt, []);
@@ -44,6 +52,8 @@
 ### Text Blob
 
 ```js
+...
+import { Buffer } from 'buffer';
 ...
     const stmt = `SELECT id, name, blobtext FROM teach;`;
     const bufText = Buffer.from(ret.values[0].blobtext);
@@ -57,6 +67,8 @@
 ### Image Blob
 
 ```js
+...
+import { Buffer } from 'buffer';
 ...
     const imagePath = "YOUR_IMAGE_PATH/favicon.png";
     const blob = await(await fetch(imagePath)).blob();
@@ -70,6 +82,8 @@
 ### Text Blob
 
 ```js
+...
+import { Buffer } from 'buffer';
 ...
     const textBuffer = Buffer.from('Hello, World!');
     const stmt = "INSERT INTO teach (id, name, blobtext)VALUES( ?,?,?);";