add Digest file stream example

_examples/digest_file_stream.md

@@ -0,0 +1,59 @@
+### Message Digest File Stream example
+
+```c
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include "crypsi.h"
+
+char* load_file(char const* path, long* length_dst);
+
+int main(int argc, char** argv) {
+    char* message = "wuriyanto";
+
+    long burger_stream_length = 0;
+
+    // load file
+    unsigned char* burger_stream = load_file("./testdata/burger.png", &burger_stream_length);
+
+    unsigned char* dst_digest;
+    int dst_digets_len;
+
+    if(crypsi_sha256(burger_stream, burger_stream_length, &dst_digest, &dst_digets_len) != 0) {
+        printf("digest error\n");
+        return -1;
+    }
+
+    printf("message len: %d\n", dst_digets_len);
+
+    printf("digest result: %s\n", dst_digest); // 6519949bc95c5f8bce78e08fffd0a8b0c7fb34514a565a09751ba58aa7be77d288a1fe17704a5b961b02f25e4186a463bd50b7ab86422d1c35fc5fdd1208c4a4
+
+    free((void*) dst_digest);
+    free((void*) burger_stream);
+
+    return 0;
+}
+
+char* load_file(char const* path, long* length_dst) {
+    char* buffer = 0;
+    long length;
+    FILE * f = fopen (path, "rb");
+
+    if (f != NULL) {
+      fseek (f, 0, SEEK_END);
+      length = ftell (f);
+      fseek (f, 0, SEEK_SET);
+      buffer = (char*) malloc((length+1)*sizeof(char));
+      if (buffer) {
+        fread(buffer, sizeof(char), length, f);
+      }
+
+      fclose(f);
+    }
+
+    buffer[length] = 0x0;
+    *length_dst = length;
+
+    return buffer;
+}
+```