getting-started: Explain why we have global keys

Make it more clear that one should not have global, hard-coded keys in
their code. We do this for example purposes only and real-world
applications should follow secure practices.

Author: Patater

getting-started/main.cpp

@@ -32,11 +32,17 @@
 0x0e, 0x42, 0x75, 0x78, 0xb5, 0x0d, 0x17, 0x4f, \
 0x6e, 0x13, 0xf4, 0xfd, 0x16, 0x30, 0x3e, 0xc7, \
 }
+/* This key is present as a global define for the purpose of example. In
+ * real-world applications, you would not have a key hardcoded in source like
+ * this. */
 #define AES_KEY \
 { \
 0xaa, 0xaa, 0xaa, 0xaa, 0xaa, 0xaa, 0xaa, 0xaa, \
 0xaa, 0xaa, 0xaa, 0xaa, 0xaa, 0xaa, 0xaa, 0xaa, \
 }
+/* This key is present as a global define for the purpose of example. In
+ * real-world applications, you would not have a key hardcoded in source like
+ * this. */
 #define RSA_KEY \
 { \
 0x30, 0x82, 0x02, 0x5e, 0x02, 0x01, 0x00, 0x02, 0x81, 0x81, 0x00, 0xaf, 0x05, \