fix platform specific ouput for multi-target project

[RLittlesII]

• added Serilog dependency
• fixing TFM for MonoAndroid

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Resolves: #19

What is the current behavior? (You can also link to an open issue here)

The packages are empty

What is the new behavior (if this is a feature change)?

The packages have the appropriate files

Does this PR introduce a breaking change?

No

Please check if the PR fulfills these requirements

• [x] The commit follows our guidelines: https://github.com/serilog/serilog#contribute
• [x] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been added / updated (for bug fixes / features)

Other information: