Update docs and repo config for new versioning approach

[thanethomson]

Related to #1018.

This PR updates the repo documentation (README, contributing guidelines, ADR docs, PR and issue templates) to (1) better reflect how we currently work, and (2) accommodate the new versioning approach.

Note that I've taken out the RFC-related documentation from the contributing guidelines because we don't do RFCs. I think it complicates the contributing guidelines as well.

Rendered versions:

• README
• Contributing guidelines
• ADR README

[thanethomson]

Changed this back to a draft PR to accommodate recent feedback on the versioning approach here.

[codecov-commenter]

Codecov Report

Merging #1020 (60a7f90) into master (9690eca) will decrease coverage by 0.0%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1020     +/-   ##
========================================
- Coverage    66.1%   66.0%   -0.1%     
========================================
  Files         209     209             
  Lines       20764   20760      -4     
========================================
- Hits        13726   13721      -5     
- Misses       7038    7039      +1     

Impacted Files	Coverage Δ
config/src/config.rs	55.0% <0.0%> (-1.5%)	⬇️
tendermint/src/proposal.rs	90.4% <0.0%> (-0.3%)	⬇️
tendermint/src/test.rs	100.0% <0.0%> (ø)
tendermint/src/node.rs	63.7% <0.0%> (+0.1%)	⬆️
abci/src/server.rs	9.8% <0.0%> (+0.2%)	⬆️
tendermint/src/vote.rs	76.6% <0.0%> (+0.3%)	⬆️
tendermint/src/signature.rs	55.0% <0.0%> (+0.5%)	⬆️
testgen/src/vote.rs	84.8% <0.0%> (+0.8%)	⬆️
testgen/src/header.rs	80.9% <0.0%> (+1.3%)	⬆️

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