Add instructions to insert metric info in DB

[snadi]

Right now, the metrics table is just used to relate charts to metrics so they can be presented in the website. Thus, it wasn't documented in this repo but this causes the scripts to fail. After creating the DB, the following needs to be run in the DB:

insert into Metric(name) value("popularity");
insert into Metric(name) value("release frequency");
insert into Metric(name) value("last discussed on so");
insert into Metric(name) value("last modification date");
insert into Metric(name) value("breaking changes");
insert into Metric(name) value("issue response");
insert into Metric(name) value("issue closing");
insert into Metric(name) value("issue classification");

[snadi]

@Hk-tang this should solve your problem. I guess we have to update the ReadMe with this but this is also related to issue #27 .. please open the issue anyways with your log and link it to this so that we know the solution if we face the same issue again