Adding instructions to update the year on copyright headers.

To avoid a common newbie mistake like the pull request rejected #3944, adding a simple instruction to the documentation Coding Convention of Bitcoin Core (doc/coding.md). The actual author of this short text (slightly modified) is Gavin Andresen sent by email to me as a tip.

Related:
- https://github.com/bitcoin/bitcoin/pull/3944#issuecomment-38467282
- https://github.com/bitcoin/bitcoin/pull/3646