perf db-export: Add brief documentation

Database Export

===============

perf tool's python scripting engine:

	tools/perf/util/scripting-engines/trace-event-python.c

supports scripts:

	tools/perf/scripts/python/export-to-sqlite.py

	tools/perf/scripts/python/export-to-postgresql.py

which export data to a SQLite3 or PostgreSQL database.

The export process provides records with unique sequential ids which allows the

data to be imported directly to a database and provides the relationships

between tables.

Over time it is possible to continue to expand the export while maintaining

backward and forward compatibility, by following some simple rules:

1. Because of the nature of SQL, existing tables and columns can continue to be

used so long as the names and meanings (and to some extent data types) remain

the same.

2. New tables and columns can be added, without affecting existing SQL queries,

so long as the new names are unique.

3. Scripts that use a database (e.g. exported-sql-viewer.py) can maintain

backward compatibility by testing for the presence of new tables and columns

before using them. e.g. function IsSelectable() in exported-sql-viewer.py

4. The export scripts themselves maintain forward compatibility (i.e. an existing

script will continue to work with new versions of perf) by accepting a variable

number of arguments (e.g. def call_return_table(*x)) i.e. perf can pass more

arguments which old scripts will ignore.

5. The scripting engine tests for the existence of script handler functions

before calling them.  The scripting engine can also test for the support of new

or optional features by checking for the existence and value of script global

variables.