Add details on weekly open day

[valentine]

There have been enquiries from potential members suggesting that they are unclear when they might be able to visit the space. The current text states:

Drop by HackerspaceSG and get to know us. Hang out for a week or two, just so that we can know you better, and so that you are sure that you really want to be part of the community.

… but some guests might be uncomfortable with the ambiguity.

One possibility is to encourage the wider community to come drop by HSG to hack on a certain day every week, most likely Saturday (if there's no major geek events happening that weekend that will attract most of HSG's regular membership).

This issue acts as a reminder to get the weekly open day going, and published on the website.