Add install instructions for ghc 8.6.5

[satnam6502]

Perhaps you don't want to have your source depend on a more recent version of GHC, but if you do then this might be helpful for other people that want to download and use Barrelfish with a more recent Haskell compiler.

    Developer's Certificate of Origin 1.1

    By making a contribution to this project, I certify that:

    (a) The contribution was created in whole or in part by me and I
        have the right to submit it under the open source license
        indicated in the file; or

    (b) The contribution is based upon previous work that, to the best
        of my knowledge, is covered under an appropriate open source
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        by me, under the same open source license (unless I am
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    (c) The contribution was provided directly to me by some other
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    Signed-off-by: Satnam Singh <satnam@google.com>

[barrelfish-bot]

Can one of the admins verify this patch?

[achreto]

Dear Satnam, thank you for your patch. Please note, we are supporting the latest Ubuntu LTS (as of now 18.04) which comes with GHC 8.0.2. We will test your changes to Paka, HamletAst and SockeyeInstantiator and merge them internally. They will be released in the next release.