Create README.md

README.md

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+# [hmac_sha256](https://github.com/h5p9sl/hmac_sha256)
+A SHA256 HMAC implementation in C/C++
+
+## Example (C++)
+```cpp
+#include "hmac_sha256.h"
+
+#include <iostream>
+#include <sstream>
+#include <cstdlib>
+#include <cstring>
+
+int main(void) {
+    static const char* testvector = "b0344c61d8db38535ca8afceafbf12b881dc20c9833da726e9376c2e32cff7";
+    uint8_t* key, *data, *out;
+    unsigned keylen, datalen, outlen;
+    std::stringstream result;
+
+    // Allocate memory
+    keylen = 20;
+    datalen = 8;
+    outlen = 32;
+    key = (uint8_t*)malloc(keylen);
+    data = (uint8_t*)malloc(datalen);
+    out = (uint8_t*)malloc(outlen);
+
+    // Initialize variables
+    strncpy((char*)data, "Hi There", datalen);
+    memset(key, 0x0b, 20);
+
+    hmac_sha256(key, keylen, data, datalen, out, outlen);
+
+    // Convert 'out' to string
+    for (unsigned i = 0; i < outlen; i++) {
+        result << std::hex << (int)out[i];
+    }
+    std::cout << result.str() << std::endl;
+    std::cout << testvector << std::endl;
+
+    // Compare result
+    if (strncmp(testvector, result.str().c_str(), result.str().length()) == 0) {
+        std::cout << "Test passed!" << std::endl;
+    } else {
+        std::cout << "Test failed." << std::endl;
+    }
+
+    return 0;
+}
+```