docs: add signal proposal to reference and fix typo

[dongsam]

Description

closes: #48

Tasks

• Add Reference docs structure
• Add first signal proposal
• Fix typo in /README.md

References

• docs: add Reference docs structure and add first signal proposal farming#148

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• Appropriate labels applied
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• Code follows the module structure standards.
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[codecov]

Codecov Report

Merging #49 (e0b310d) into master (d6677f3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #49   +/-   ##
=======================================
  Coverage   80.54%   80.54%           
=======================================
  Files          12       12           
  Lines         293      293           
=======================================
  Hits          236      236           
  Misses         42       42           
  Partials       15       15           

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[dongsam]

@kogisin I just copy docs from tendermint/farming#148 and fix farming words to budget and remove signal words because there would be not only signal proposal but also upgrade proposals on the proposals/ directory.
I missed that when reviewing tendermint/farming#148.

[jaybxyz]

LGTM