Update README with placeholder in Motivation section

[fredbi]

Added placeholder 'xxx' in the Motivation section.c

---

name: 'Please selct: 🆕 New feature or enhancement|🔧 Bug fix'|📃 Documentation update'
about: Help us enhance go-openapi by adding or improving features
title: ''
labels: [enhancement]
assignees: ''

---

Short description

Fixes

Full description

Checklist

• I have signed all my commits with my name and email (see DCO. This does not require a PGP-signed commit
• I have rebased and squash my work, so only one commit remains
• I have added tests to cover my changes.
• I have properly enriched go doc comments in code.
• I have properly documented any breaking change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2ec60ac) to head (0480539).

Additional details and impacted files

@@            Coverage Diff            @@
##            master        #3   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines            4         4           
=========================================
  Hits             4         4           

Flag	Coverage Δ
oldstable-macos-latest	100.00% <ø> (ø)
oldstable-ubuntu-latest	100.00% <ø> (ø)
oldstable-windows-latest	100.00% <ø> (ø)
stable-macos-latest	100.00% <ø> (ø)
stable-ubuntu-latest	100.00% <ø> (ø)
stable-windows-latest	100.00% <ø> (ø)

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