Update README, add deprecation warning

ts_src/transaction_builder.ts

@@ -146,6 +146,13 @@ export class TransactionBuilder {
     this.__TX = new Transaction();
     this.__TX.version = 2;
     this.__USE_LOW_R = false;
+    console.warn(
+      'Deprecation Warning: TransactionBuilder will be removed in the future. ' +
+        '(v6.x.x or later) Please use the Psbt class instead. Examples of usage ' +
+        'are available in the transactions-psbt.js integration test file on our ' +
+        'Github. A high level explanation is available in the psbt.ts and psbt.js ' +
+        'files as well.',
+    );
   }
 
   setLowR(setting?: boolean): boolean {