add basic example for compiling upkr to a c library

c_library/upkr.h

@@ -0,0 +1,25 @@
+#ifndef UPKR_H_INCLUDED
+
+#include <stddef.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// input_buffer/input_size: input data to compress
+// output_buffer/output_buffer_size: buffer to compress into
+// compression_level: 0-9
+// returns the size of the compressed data, even if it didn't fit into the output buffer
+size_t upkr_compress(void* output_buffer, size_t output_buffer_size, void* input_buffer, size_t input_size, int compression_level);
+
+// input_buffer/input_size: compressed data
+// output_buffer/output_buffer_size: buffer to uncompress into
+// return value:
+//  >= 0 : size of uncompressed data, even if it didn't fit into the output buffer
+//  < 0  : input data corrupt, unable to decompress
+ptrdiff_t upkr_uncompress(void* output_buffer, size_t output_buffer_size, void* input_buffer, size_t input_size);
+
+#ifdef __cplusplus
+}
+#endif
+#endif