[SDK-3694] README redesign (#617)

Author: jimmyjames

EXAMPLES.md

@@ -0,0 +1,130 @@
+# Examples using java-jwt
+
+* [Inspecting a DecodedJWT](#inspecting-a-decodedjwt)
+* [DateTime Claim Validation](#datetime-claim-validation)
+* [Using custom claims](#using-custom-claims)
+* [Using a KeyProvider](#using-a-keyprovider)
+
+## Inspecting a DecodedJWT
+
+The successful verification of a JWT returns a `DecodedJWT`, from which you can obtain its contents.
+
+```java
+DecodedJWT jwt = JWT.require(algorithm)
+        .build()
+        .verify("a.b.c");
+
+// standard claims can be retrieved through first-class methods
+String subject = jwt.getSubject();
+String aud = jwt.getAudience();
+// ...
+
+// custom claims can also be obtained
+String customStringClaim = jwt.getClaim("custom-string-claim").asString();
+```
+
+When retrieving custom claims, a [Claim](https://javadoc.io/doc/com.auth0/java-jwt/latest/com/auth0/jwt/interfaces/Claim.html) is returned, which can then be used to obtain the value depending on the value's underlying type.
+
+## DateTime Claim Validation
+
+A JWT token may include DateNumber fields that can be used to validate that:
+
+* The token was issued in a past date `"iat" < NOW`
+* The token hasn't expired yet `"exp" > NOW`
+* The token can already be used. `"nbf" < NOW`
+
+When verifying a JWT, the standard DateTime claims are validated by default. A `JWTVerificationException` is thrown if any of the claim values are invalid.
+
+To specify a **leeway** in which the JWT should still be considered valid, use the `acceptLeeway()` method in the `JWTVerifier` builder and pass a positive seconds value. This applies to every item listed above.
+
+```java
+JWTVerifier verifier = JWT.require(algorithm)
+    .acceptLeeway(1) // 1 sec for nbf, iat and exp
+    .build();
+```
+
+You can also specify a custom value for a given DateTime claim and override the default one for only that claim.
+
+```java
+JWTVerifier verifier = JWT.require(algorithm)
+    .acceptLeeway(1)   //1 sec for nbf and iat
+    .acceptExpiresAt(5)   //5 secs for exp
+    .build();
+```
+
+If you need to test this behavior in your application, cast the `Verification` instance to a `BaseVerification` to gain visibility of the `verification.build()` method that accepts a `java.time.Clock`. e.g.:
+
+```java
+BaseVerification verification = (BaseVerification) JWT.require(algorithm)
+    .acceptLeeway(1)
+    .acceptExpiresAt(5);
+private final Clock mockNow = Clock.fixed(Instant.ofEpochSecond(1477592), ZoneId.of("UTC"));    
+JWTVerifier verifier = verification.build(clock);
+```
+
+## Using custom claims
+
+### JWT creation
+A JWT can be built with custom payload and header claims, by using the `withHeader` and `withClaim` methods.
+
+```java
+String jwt = JWT.create()
+        .withHeader(headerMap)
+        .withClaim("string-claim", "string-value")
+        .withClaim("number-claim", 42)
+        .withClaim("bool-claim", true)
+        .withClaim("datetime-claim", Instant.now())
+        .sign(algorithm);
+```
+
+See the [JavaDoc](https://javadoc.io/doc/com.auth0/java-jwt/latest/com/auth0/jwt/JWTCreator.Builder.html) for all available custom claim methods.
+
+### JWT verification
+
+You can also verify a JWT's custom claims:
+
+```java
+JWTVerifier verifier = JWT.require(algorithm)
+        .withClaim("number-claim", 123)
+        .withClaimPresence("some-claim-that-just-needs-to-be-present")
+        .withClaim("predicate-claim", (claim, decodedJWT) -> "custom value".equals(claim.asString()))
+        .build();
+DecodedJWT jwt = verifier.verify("my.jwt.token");
+```
+
+See the [JavaDoc](https://javadoc.io/doc/com.auth0/java-jwt/latest/com/auth0/jwt/JWTVerifier.BaseVerification.html) for all available custom claim verification methods.
+
+## Using a KeyProvider
+
+A `KeyProvider` can be used to obtain the keys needed for signing and verifying a JWT. How these keys are constructed are beyond the scope of this library, but the [jwks-rsa-java](https://github.com/auth0/jwks-rsa-java) library provides the ability to obtain the public key from a JWK.
+The example below demonstrates this for the RSA algorithm (`ECDSAKeyProvider` can be used for ECDSA).
+
+```java
+JwkProvider provider = new JwkProviderBuilder("https://samples.auth0.com/")
+        .cached(10, 24, TimeUnit.HOURS)
+        .rateLimited(10, 1, TimeUnit.MINUTES)
+        .build();
+final RSAPrivateKey privateKey = // private key
+final String privateKeyId = // private key ID
+
+RSAKeyProvider keyProvider = new RSAKeyProvider() {
+    @Override
+    public RSAPublicKey getPublicKeyById(String kid) {
+        return (RSAPublicKey) jwkProvider.get(kid).getPublicKey();
+    }
+
+    @Override
+    public RSAPrivateKey getPrivateKey() {
+        // return the private key used 
+        return rsaPrivateKey;
+    }
+
+    @Override
+    public String getPrivateKeyId() {
+        return rsaPrivateKeyId;
+    }
+};
+
+Algorithm algorithm = Algorithm.RSA256(keyProvider);
+//Use the Algorithm to create and verify JWTs.
+```