contributing.rst: Add more detail.
contributing.rst: Add more detail.

Make it easier for beginners to start working with the cilium toolchain.

Signed-off-by: Jarno Rajahalme <0f1ab0ac7dffd9db21aa539af2fd4bb04abc3ad4@covalent.io>

cilium/cilium,tklauser/cilium,eloycoto/cilium,michi-covalent/cilium,tgraf/cilium,mskarbek/cilium,gumpt/cilium,tklauser/cilium,gumpt/cilium,eloycoto/cilium,tgraf/cilium,michi-covalent/cilium,cilium/cilium,mskarbek/cilium,gumpt/cilium,gumpt/cilium,scanf/cilium,scanf/cilium,scanf/cilium,mskarbek/cilium,cilium/cilium,scanf/cilium,tgraf/cilium,eloycoto/cilium,eloycoto/cilium,eloycoto/cilium,cilium-team/cilium,mskarbek/cilium,tklauser/cilium,scanf/cilium,scanf/cilium,cilium-team/cilium,michi-covalent/cilium,eloycoto/cilium,mskarbek/cilium,cilium/cilium,gumpt/cilium,tgraf/cilium,tgraf/cilium,michi-covalent/cilium,tklauser/cilium,scanf/cilium,gumpt/cilium,tgraf/cilium,eloycoto/cilium,cilium/cilium,mskarbek/cilium,gumpt/cilium,michi-covalent/cilium,tklauser/cilium