Added information about legacy version 1.0.0

[brunats]

- What I did

Added a section on the deprecated version 1.0.0.
It contains a link to the legacy branch and information about the type of changes that can be made (bug fixes only).

- How to verify it
See the readme.md file.

- Description for the changelog

Adds information to the readme about deprecated version ritchie-legacy1.0.0

[codecov-commenter]

Codecov Report

Merging #353 into ritchie-2.0.0 will increase coverage by 0.95%.
The diff coverage is 73.38%.

@@                Coverage Diff                @@
##           ritchie-2.0.0     #353      +/-   ##
=================================================
+ Coverage          65.21%   66.17%   +0.95%     
=================================================
  Files                 78       78              
  Lines               2550     2572      +22     
=================================================
+ Hits                1663     1702      +39     
+ Misses               725      701      -24     
- Partials             162      169       +7     

Impacted Files	Coverage Δ
pkg/cmd/build_formula.go	0.00% <0.00%> (ø)
pkg/cmd/root.go	0.00% <0.00%> (ø)
pkg/credential/finder.go	68.42% <ø> (ø)
pkg/formula/builder/bat.go	0.00% <0.00%> (ø)
pkg/formula/builder/local.go	72.54% <ø> (ø)
pkg/formula/formula.go	90.90% <ø> (ø)
pkg/formula/runner/post_run.go	50.00% <ø> (ø)
pkg/cmd/list_credential.go	80.00% <50.00%> (ø)
pkg/cmd/formulas.go	87.87% <57.14%> (-2.45%)	⬇️
pkg/credential/settings.go	82.64% <60.86%> (-3.77%)	⬇️
... and 19 more

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