Add function details in readme

Author: haldarmahesh

README.md

@@ -10,30 +10,46 @@ This plugin exposes the following methods:
 * `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
+### 1) 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: 
+#### defination
+
+```dart
+static Future<String> getPublicKey()
+
+```
+#### usage: 
 
 ```dart
 final String rsaPublicKey = await Cipher.getPublicKey();
 ```
 
-### sign
+### 2) sign
 This function returns a signature which is signed by the platoform's private key.
+#### defination
 
-usage: 
 ```dart
-final String signature = await Cipher.sign('some plain data');
+static Future<String> sign(String plainData)
+```
 
+#### usage: 
+```dart
+final String signature = await Cipher.sign('some plain data');
 ```
 
-### verify
+### 3) 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.
 
+#### defination
+
+```dart
+static Future<bool> verify(String plainText, String signature)
+```
+
 usage:
 ```dart
 final bool result = await Cipher.verify('somePassword', 'SIGNATURE-XX');