Enhance changelog

[RadicalZephyr]

I added an "Unreleased" section and linked the section headers to appropriate pages on the layabout github.

[coveralls]

Pull Request Test Coverage Report for Build 120

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 115:	0.0%
Covered Lines:	114
Relevant Lines:	114

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💛 - Coveralls

[reillysiemens]

@RadicalZephyr: When generating the Sphinx documentation with these changes some warnings are thrown:

reading sources... [100%] why                                                                                                                                                                 
../CHANGELOG.rst:5: WARNING: Title underline too short.

Unreleased_
----------
../CHANGELOG.rst:13: WARNING: Title underline too short.

v1.0.0_ (2018-06-18)
-------------------
../CHANGELOG.rst:13: WARNING: Title underline too short.

v1.0.0_ (2018-06-18)
-------------------

The title underlines need to be just a little bit longer.

[reillysiemens]

Another welcome change. Thanks, @RadicalZephyr! 😁