Add pbuilder mirror instructions.

The first time running `m` gave me a screen asking to set the mirror
site for pbuilder. This clarifies that the suggested debian.org mirror
works.

Change-Id: I09bf52f9d1a7d5d2a145c978c3a1a745cf88f164
1 file changed
tree: 8a8d8c7d0ee52fef4622c6333fa9e1bd8b1aa209
  1. Contributing.md
  2. GettingStarted.md
  3. License.md