Simplify init scheme; update & clarify the README

Now the setup is to add to both rc and profile:
1) set PYENV_ROOT
   (can do it unconditionally -- since if you change it,
   you need to update all places anyway since any of them can be run first)
2) Add `pyenv` to PATH if not already there
3) eval "$(pyenv init -)"

Not a breaking change, old setup will continue to work.

.github/workflows/pyenv_tests.yml

@@ -24,7 +24,12 @@ jobs:
               # xz-utils tk-dev libffi-dev liblzma-dev python-openssl git
       # https://github.com/pyenv/pyenv#installation
       - run: |
-          if test "$RUNNER_OS" == "macOS"; then brew install coreutils; fi
+          if test "$RUNNER_OS" == "macOS"; then
+            brew install coreutils fish
+          elif [[ $(lsb_release -sr | awk -F. '{print $1}') -ge 20 ]]; then
+            # Ubuntu 18 has fish 2 which lacks many features that facilitate testing
+            sudo apt install fish -yq
+          fi
       - run: pwd
       - env:
           PYENV_ROOT: /home/runner/work/pyenv/pyenv