merge tjoneslo: Add a security.md document file to the base of the repo
merge tjoneslo: Add a security.md document file to the base of the repo

Merge pull request #1490 from tjoneslo/sql-layer/security.md
https://github.com/FoundationDB/sql-layer/pull/1490

jaytaylor/sql-layer,jaytaylor/sql-layer,relateiq/sql-layer,qiuyesuifeng/sql-layer,qiuyesuifeng/sql-layer,relateiq/sql-layer,shunwang/sql-layer-1,wfxiang08/sql-layer-1,wfxiang08/sql-layer-1,qiuyesuifeng/sql-layer,ngaut/sql-layer,relateiq/sql-layer,shunwang/sql-layer-1,relateiq/sql-layer,ngaut/sql-layer,shunwang/sql-layer-1,wfxiang08/sql-layer-1,ngaut/sql-layer,shunwang/sql-layer-1,wfxiang08/sql-layer-1,jaytaylor/sql-layer,ngaut/sql-layer,jaytaylor/sql-layer,qiuyesuifeng/sql-layer