Add comment about heatshrink.c builds breaking due to err.h or getopt.h.

A couple people have opened issues because they can't get this file
to build for Arduinos, etc., but it isn't supposed to be. Add a comment
comment explaining why right next to those header includes to clarify.

Closes #48, #61.

CHANGELOG.md

@@ -16,6 +16,9 @@ Fixed fallthrough in getopt 'h'.
 
 Added `CHANGELOG.md`.
 
+Added comment to help explain build error when attempting to build the
+command line interface for embedded systems without getopt.h/err.h.
+
 Use constant rather than bare int 4096 for buffer size.
 
 Fixed fallthrough compiler warning.

src/heatshrink.c

@@ -5,7 +5,6 @@
 #include <assert.h>
 #include <string.h>
 #include <fcntl.h>
-#include <getopt.h>
 
 #include "heatshrink_encoder.h"
 #include "heatshrink_decoder.h"
@@ -21,6 +20,18 @@
 #define LOG(...) /* NO-OP */
 #endif
 
+/* If you're getting a build error because getopt.h or err.h are not
+ * available when compiling for an embedded platform, you're probably
+ * misundersting how this is supposed to be used.
+ *
+ * The encoder/decoder source files are for a library for embedded
+ * devices, but this file is for a command line program for a Unix-like
+ * environment, to work with data from embedded systems using the
+ * heatshrink library, experiment with how effectively different
+ * settings compress test data, etc. It is not supposed to be built for
+ * embedded devices, which are unlikely to have those headers. */
+#include <getopt.h>
+
 #if _WIN32
 #include <errno.h>
 #define HEATSHRINK_ERR(retval, ...) do { \