Add sample use of libscrypt.a and docs

BUILDING

@@ -15,6 +15,15 @@ Official scrypt tarball releases should build and run on any IEEE Std 1003.1
   3. Provides /dev/urandom.
 
 
+libscrypt
+---------
+
+To install the development library, run:
+
+    ./configure --enable-libscrypt
+    make install
+
+
 Platform-specific notes
 -----------------------
 

README.md

@@ -113,6 +113,16 @@ int crypto_scrypt(const uint8_t *, size_t, const uint8_t *, size_t, uint64_t,
     uint32_t, uint32_t, uint8_t *, size_t);
 ```
 
+There is a sample of using `crypto_scrypt()` in `tests/libscrypt`.  If you
+installed `libscrypt`, you can compile that file and run the binary:
+
+```
+$ cd tests/libscrypt/
+$ cc sample-libscrypt.c -lscrypt
+$ ./a.out
+crypto_scrypt(): success
+```
+
 
 Building
 --------

tests/libscrypt/sample-libscrypt.c

@@ -0,0 +1,33 @@
+#include <stdint.h>
+#include <stdio.h>
+#include <string.h>
+
+#include "crypto_scrypt.h"
+
+/* Parameters controlling memory usage and CPU time. */
+#define N 16384
+#define r 8
+#define p 1
+
+/* How much data should scrypt return? */
+#define OUTPUT_BUFLEN 8
+
+int main()
+{
+	const char * passwd = "hunter2";
+	const char * salt = "DANGER -- this should be a random salt -- DANGER";
+	uint8_t output[OUTPUT_BUFLEN];
+	int exitcode;
+
+	exitcode = crypto_scrypt((const uint8_t *)passwd, strlen(passwd),
+	    (const uint8_t*)salt, strlen(salt), N, r, p,
+	    output, OUTPUT_BUFLEN);
+
+	/* Notify user of success / failure. */
+	if (exitcode == 0)
+		printf("crypto_scrypt(): success\n");
+	else
+		printf("crypto_scrypt(): failure %i\n", exitcode);
+
+	return (exitcode);
+}