notes/postgres/out_of_memory.md

logs from trying to start with not enough memory:
```
207716 2020-02-11 11:33:02 EST [2969]: [1-1] user=,db=,app=,client=,remote=LOG:  starting PostgreSQL 12.1 (Ubuntu 12.1-1.pgdg18.04+1) on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0, 64-bit
207717 2020-02-11 11:33:02 EST [2969]: [2-1] user=,db=,app=,client=,remote=LOG:  listening on IPv4 address "0.0.0.0", port 5030
207718 2020-02-11 11:33:02 EST [2969]: [3-1] user=,db=,app=,client=,remote=LOG:  listening on IPv6 address "::", port 5030
207719 2020-02-11 11:33:04 EST [2969]: [4-1] user=,db=,app=,client=,remote=LOG:  listening on Unix socket "/var/run/postgresql/.s.PGSQL.5030"
207720 2020-02-11 11:33:06 EST [2969]: [5-1] user=,db=,app=,client=,remote=FATAL:  could not map anonymous shared memory: Cannot allocate memory
207721 2020-02-11 11:33:06 EST [2969]: [6-1] user=,db=,app=,client=,remote=HINT:  This error usually means that PostgreSQL's request for a shared memory segment exceeded available memory, swap space, or huge pages. To reduce the request size (currently 4423614464 bytes       ), reduce PostgreSQL's shared memory usage, perhaps by reducing shared_buffers or max_connections.
207722 2020-02-11 11:33:06 EST [2969]: [7-1] user=,db=,app=,client=,remote=LOG:  database system is shut down
207723 pg_ctl: could not start server
207724 Examine the log output.
```