Add preliminary members base documentation.

MEMBERS.md

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+Members Base
+============
+
+This file documents the internal structure of the members information
+storage.
+
+Members Directory
+-----------------
+
+Files in this directory with file names being four decimal digits are
+member files. The file name is the member id.
+
+Symlinks to the member files serve as human-readable aliases.
+
+The implementation handles situation when the human-readable name is
+the member file and the symlink is the four-digit member id.
+
+Member File Preprocessing
+-------------------------
+
+Member files are read line-by-line.
+
+Comments start with the ```#``` character and continue until the end
+of particular line.
+
+Comments are stripped from each line before parsing.
+
+Member File Grammar
+-------------------
+
+Member File Schema
+------------------