Update README.md

Author: haldarmahesh

README.md

@@ -4,7 +4,40 @@ This plugin helps you by generating the assymetric RSA key pair.
 
 The keys are generated and persisted in android/ios keystore.
 
+## Using plugin
 This plugin exposes the following methods:
 * `getPublicKey`: returns the RSA public key
 * `encrypt`: encrypts the plain text using private key from the key pair
 * `verify`: verifies a signature and returns a boolean if the signature is signed by the same key pair.
+
+### getPublicKey
+This function is used to get the RSA public key which is store the android and ios key store.
+This key pair is persisted in key store, i.e it is generated once, and always re used for signing and verification.
+
+usage: 
+
+```dart
+final String rsaPublicKey = await Cipher.getPublicKey();
+```
+
+### sign
+This function returns a signature which is signed by the platoform's private key.
+
+usage: 
+```dart
+final String signature = await Cipher.sign('some plain data');
+
+```
+
+### verify
+This function returns a boolean, and takes a plain data and signature.
+
+This function verifies the signature against the plain data, it returns `true` if the signature is produced by the same device's private key.
+
+usage:
+```dart
+final bool result = await Cipher.verify('somePassword', 'SIGNATURE-XX');
+```
+
+The above will checks if the `SIGNATURE-XX` matches the `somePassword` or not. 
+