Add RAM Requirements to README

[lkowalick]

Some community users running filecoin on AWS instances with 4GB of RAM notice that the daemon will run out of memory. Upping the memory to 8GB resolves the problem, so we are adding this in as a system requirement.

Resolves #2161

[sandjj]

I have the same problem. My server has 16G memory, but after running for 1 or 2 weeks, it also reported memory allocation errors.

[root@ipfs-svr94 ~]# tail -f logfile
00:13:42.070 INFO autonat: Discovered AutoNAT peer QmXq6XEYeEmUzBFuuKbVEGgxEpVD4xbSkG2Rhek6zkFMp4 notify.go:33
00:13:42.384 INFO consensus.: [TIMER] DefaultProcessor.ApplyMessage CID: zDPWYqFCzdxKR67uAfyz8H1TAaqAYAPKsqVWyfpvJoFysADHcTLG - elapsed time: 2.575s processor.go:267
00:13:42.792 INFO consensus.: [TIMER] DefaultProcessor.ApplyMessage CID: zDPWYqFD7c4afRd8B4hxVyg5eYastqDo2QtcSuYVvJKQyJtxDQPQ - elapsed time: 830ms processor.go:267
Mar 12 00:13:43.489 INFO setting tau/aux, layer: 0, place: storage-proofs/src/layered_drgporep.rs:437 storage_proofs::layered_drgporep, root: storage-proofs
00:13:44.302 INFO consensus.: [TIMER] DefaultProcessor.ApplyMessage CID: zDPWYqFCrh5RaxgGoZR62q1NUeCZtMtUTGFHhzNitjHecPhYQ4cU - elapsed time: 1.908s processor.go:267
00:13:46.221 INFO autonat: Discovered AutoNAT peer QmZGDLdQLUTi7uYTNavKwCd7SBc5KMfxzWxAyvqRQvwuiV notify.go:33
00:13:46.454 INFO consensus.: [TIMER] DefaultProcessor.ApplyMessage CID: zDPWYqFD5jHUieVQXerMmSiWXker6iDSUqg19R2ZZSh1qVro59hC - elapsed time: 3.655s processor.go:267
Mar 12 00:13:49.790 INFO setting tau/aux, layer: 1, place: storage-proofs/src/layered_drgporep.rs:437 storage_proofs::layered_drgporep, root: storage-proofs
Mar 12 00:13:54.939 INFO setting tau/aux, layer: 2, place: storage-proofs/src/layered_drgporep.rs:437 storage_proofs::layered_drgporep, root: storage-proofs
memory allocation of 536870880 bytes failed^C
[root@ipfs-svr94 ~]#
[root@ipfs-svr94 ~]# uname -a
Linux ipfs-svr94 3.10.0-957.el7.x86_64 #1 SMP Thu Nov 8 23:39:32 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
[root@ipfs-svr94 ~]# cat /etc/redhat-release
CentOS Linux release 7.6.1810 (Core)
[root@ipfs-svr94 ~]#
[root@ipfs-svr94 ~]# free -m
total used free shared buff/cache available
Mem: 15633 266 15007 36 358 14950
Swap: 7935 871 7064
[root@ipfs-svr94 ~]#

I wonder if a parameter for maximum memory usage to the filecoin can be provided in configuration file.