Add note about updating copyright notes to the manual

[craffael]

• I've checked this Git style guide. Yes
• I've checked this CMake style guide. Yes
• My change will work with CMake 3.2 (minimum requirement for Hunter). Yes
• I will try to keep this pull request as small as possible and will try not to mix unrelated features. *Yes

As @NeroBurner pointed out in #600 one should update the copyright notices when a new package is created. This PR points this out to the user in the guide to create a new package.

[NeroBurner]

@rbsheth could you have a look at this. I'm not sure what exactly the copyright notice in the new packages should be

[NeroBurner]

Released: https://github.com/cpp-pm/hunter/releases/tag/v0.26.0