wip - using ECDSA tutorial

docs/low-level/USING_ECDSA.md

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+# Low-level: Making Use of ECDSA with Public and Private Keys
+
+In this tutorial, we will learn how to use ECDSA with asymmetric public and private key pairs. Note, this is a low-level tutorial and should only be used if you would like to add advanced features to your applications. 
+
+## Getting Started
+
+First, you will want to make sure you have installed the required dependencies.
+
+```ts
+import { PrivateKey, Utils } from '@bsv/sdk'
+```
+
+```ts
+const privateKey = PrivateKey.fromRandom()
+const message = Utils.toArray('Message to sign!')
+const signature = privateKey.sign(message)
+const valid = privateKey.verify(message, signature)
+// console.log(valid) --> true
+```