docs: add Reference docs for signal proposals record

[dongsam]

Summary

As there is going to be a signal proposal to adopt the budget and farming modules in Cosmos Hub in the near future, it is suggested to document a record of signal proposals in docs folder so that anyone can track of the history.

Proposal

Tasks

• Add Reference folder and provide skeleton structure
• Add signal proposal to adopt budget and farming modules

References

• https://github.com/allinbits/cosmos-cash/blob/main/docs/Reference/README.md
• docs: add Reference docs for signal proposals record farming#147
• docs: add Reference docs structure and add first signal proposal farming#148

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