Add some documentation.

docs/padding.txt

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+https://stackoverflow.com/questions/11642210/computing-padding-required-for-n-byte-alignment
+
+There is a faster way to compute the padding, if the alignment is a power of two
+(2,4,8,...). The following runs because binary & is similar to % for powers of
+two: %(2^x) and &(2^x-1) do the same for positive numbers. Attention: & will
+delete the sign bit and therefore always returns the positive modulo result.
+
+So (4 - (string_length & 3)) & 3 will do the same as
+(4 - (string_length % 4)) % 4. Using the positive modulo property this can be
+simplified to (-string_length) & 3!
+
+
+If you wanna add that result to the size you can even do more optimizations:
+
+padded_length = (string_length + 3) & ~3 Semantically this 'rounds up' the
+number to the padding size of 4.