iOS/ARM64: Add build instructions.

Thanks to Vyacheslav Egorov.

doc/install.html

@@ -452,11 +452,18 @@ much slower than the JIT compiler. Please complain to Apple, not me.
 Or use Android. :-p
 </p>
 <pre class="code">
+# iOS/ARM (32 bit)
 ISDKP=$(xcrun --sdk iphoneos --show-sdk-path)
 ICC=$(xcrun --sdk iphoneos --find clang)
 ISDKF="-arch armv7 -isysroot $ISDKP"
 make HOST_CC="clang -m32 -arch i386" CROSS="$(dirname $ICC)/" \
      TARGET_FLAGS="$ISDKF" TARGET_SYS=iOS
+
+# iOS/ARM64
+ISDKP=$(xcrun --sdk iphoneos --show-sdk-path)
+ICC=$(xcrun --sdk iphoneos --find clang)
+ISDKF="-arch arm64 -isysroot $ISDKP"
+make CROSS="$(dirname $ICC)/" TARGET_FLAGS="$ISDKF" TARGET_SYS=iOS
 </pre>
 
 <h3 id="consoles">Cross-compiling for consoles</h3>