Move build instructions from old wiki

The wiki hosted at stellar.org (https://wiki.stellar.org/) is in the process of being shut down in lieu of other sources of information such as [on the website](https://www.stellar.org/learn/) or on the [docs repo](https://github.com/stellar/docs).

I converted the mediawiki markup (https://wiki.stellar.org/Building_Stellard) to markdown here. It is a quick conversion without much content changes.