Update readme with srtp patching instructions

This is needed to compile with OS X 10.8 support

README.md

@@ -59,6 +59,22 @@ Add `static` at the line 286 in `third_party/srtp/crypto/cipher/aes_icm.c`:
        /* fill buffer with new keystream */
        v128_copy(&c->keystream_buffer, &c->counter);
 
+Add `static` at the line 127 in `third_party/srtp/crypto/math/datatypes.c`:
+
+    Index: third_party/srtp/crypto/math/datatypes.c
+    ===================================================================
+    --- third_party/srtp/crypto/math/datatypes.c	(revision 4641)
+    +++ third_party/srtp/crypto/math/datatypes.c	(working copy)
+    @@ -124,7 +124,7 @@
+       return bit_string;
+     }
+
+    -inline int
+    +static inline int
+     hex_char_to_nibble(uint8_t c) {
+       switch(c) {
+       case ('0'): return 0x0;
+
 Configure and build pjsip:
 
     $ CFLAGS="-mmacosx-version-min=10.8" ./configure