README.md: Fix for Homebrew and Conda
README.md: Fix for Homebrew and Conda

In README we provide instructions for how to build Ledger instead of
how to use Ledger. So the sections for Homebrew and Conda are
currently inconsistent with others.

Therefore, we update the two sections to provide installation
instructions for build dependencies.

[ci skip]

ledger/ledger,ledger/ledger,ledger/ledger,afh/ledger,afh/ledger,afh/ledger,ledger/ledger,afh/ledger,afh/ledger,ledger/ledger