Merge pull request #797 from braydonf/docs/public-key

Docs: Added PublicKey general documentation

docs/PublicKey.md

@@ -0,0 +1,31 @@
+# Public Key
+
+Represents a bitcoin public key and is needed to be able to receive bitcoin, as is usually represented as a bitcoin [Address](Address.md), see the official [Bitcoin Wiki](https://en.bitcoin.it/wiki/Technical_background_of_version_1_Bitcoin_addresses). A PublicKey in Bitcore is an immutable object and can be instantiated from a [Point](Crypto.md), string, [PrivateKey](PrivateKey.md), Buffer and a [BN](Crypto.md).
+
+## Instantiate a Public Key
+
+Here is how to instantiate a public key:
+
+```javascript
+
+var privateKey = new PrivateKey();
+
+// from a private key
+var publicKey = new PublicKey(privateKey);
+
+// from a der hex encoded string
+var publicKey2 = new PublicKey('02a1633cafcc01ebfb6d78e39f687a1f0995c62fc95f51ead10a02ee0be551b5dc');
+
+```
+
+## Validating a Public Key
+
+A public key point should be on the [secp256k1](https://en.bitcoin.it/wiki/Secp256k1) curve, instantiating a new PublicKey will validate this and will throw an error if it's invalid. To check that a public key is valid:
+
+```javascript
+if (PublicKey.isValid('02a1633cafcc01ebfb6d78e39f687a1f0995c62fc95f51ead10a02ee0be551b5dc')){
+  // valid public key
+}
+```
+
+Note: It's important to note that there are two possible ways to represent public key, the standard is *compressed* and includes the x value and parity (as represented above in the documentation). There is also a longer version that is *uncompressed* which includes both x and y values, and using this can generate a different bitcoin address, so it's important to note this possibility, however it's discouraged to be used.