64b27c46e4c21fc7902a69d8ddb6791ef417c4af docs: add reduce-memory.md (fanquake)
Pull request description:
Following some discussion in https://github.com/bitcoin-core/docs/issues/50, this adds Wladimir's [reducing bitcoind memory usage gist](https://gist.github.com/laanwj/efe29c7661ce9b6620a7) to `/doc`.
The conclusion seemed to be that if the main repo already has [reduce-traffic.md](https://github.com/bitcoin/bitcoin/blob/master/doc/reduce-traffic.md), then we could also add `reduce-memory.md`.
ACKs for top commit:
practicalswift:
ACK 64b27c46e4c21fc7902a69d8ddb6791ef417c4af
hebasto:
ACK 64b27c46e4c21fc7902a69d8ddb6791ef417c4af, I have reviewed the changes and they look OK, I agree they can be merged. Also a link from `/doc/README.md` has been tested.
jonasschnelli:
ACK 64b27c46e4c21fc7902a69d8ddb6791ef417c4af
Tree-SHA512: 0ab3035403e5145cfe33c29990a8d082df834ac6602b4ad6bfa821523d57e8451f0cde3017fbf3c2c4e0b34941b6374909d11d27f9598e211bbc14accd487be1
