docs: add demo for budget with farming

[jaybxyz]

Description

closes: #121

Tasks

• Add budget_with_farming.md demo docs
• Change the file name from demo_2021_08_06.md to mvp_legacy.md

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• Appropriate labels applied
• Targeted PR against correct branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Re-reviewed Files changed in the Github PR explorer
• Review Codecov Report in the comment section below once CI passes

[codecov]

Codecov Report

Merging #140 (a1d2ba9) into master (9c24552) will not change coverage.
The diff coverage is n/a.

❗ Current head a1d2ba9 differs from pull request most recent head 97c4dde. Consider uploading reports for the commit 97c4dde to get more accurate results

@@           Coverage Diff           @@
##           master     #140   +/-   ##
=======================================
  Coverage   69.71%   69.71%           
=======================================
  Files          25       25           
  Lines        2262     2262           
=======================================
  Hits         1577     1577           
  Misses        533      533           
  Partials      152      152           

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