Give higher priority to partial-unification fix

[LukaJCB]

After answering questions about relating to partial-unification, I think we should reprioritize its standing in the Readme.md. This PR mentions the fix up front instead of a side-note further down below. WDYT? :)

[codecov-io]

Codecov Report

Merging #1946 into master will not change coverage.
The diff coverage is n/a.

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  Hits         4257     4257           
  Misses        197      197

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[tpolecat]

This is an improvement but I think I would probably still miss it ... when I look at a README like this I tend to gloss over everything other than the code blocks. Don't know if anyone else does this. I think I would be more likely to notice something like

Cats relies on improved type inference via the fix for SI-2112, which is not enabled by default. For Scala 2.11.9 or later you should add the following to your build:

scalacOptions += "-Ypartial-unification"

Or, if you need to support older versions of Scala you can use the sbt-partial-unification plugin which extends support back through Scala 2.10.6 or later:

addSbtPlugin("org.lyranthe.sbt" % "partial-unification" % "1.1.0")

[kailuowang]

Good idea @tpolecat

[LukaJCB]

Thanks @tpolecat, I definitely agree with your suggestion! :)
I added something along the lines, WDYT?

[tpolecat]

👍