[MRG] Minor updates to release procedure

[ctb]

Updates to release procedure based on following release notes for v3.4.0 (#1100):

• replace ssh url with origin
• note travis build generates artifacts
• rust install / update instructions

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[ctb]

Ready for review & merge @luizirber

[codecov]

Codecov Report

Merging #1102 into master will increase coverage by 9.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1102      +/-   ##
==========================================
+ Coverage   83.86%   92.89%   +9.03%     
==========================================
  Files          98       74      -24     
  Lines        9108     5726    -3382     
==========================================
- Hits         7638     5319    -2319     
+ Misses       1470      407    -1063     

Flag	Coverage Δ
#rusttests	?

Impacted Files	Coverage Δ
sourmash/signature.py	92.85% <0.00%> (-0.27%)	⬇️
sourmash/_minhash.py	94.82% <0.00%> (-0.26%)	⬇️
sourmash/command_compute.py	97.44% <0.00%> (-0.20%)	⬇️
sourmash/sbtmh.py	89.71% <0.00%> (-0.19%)	⬇️
sourmash/nodegraph.py	93.57% <0.00%> (-0.18%)	⬇️
sourmash/sbt_storage.py	92.00% <0.00%> (-0.12%)	⬇️
sourmash/index.py	97.61% <0.00%> (-0.09%)	⬇️
sourmash/sbt.py	88.79% <0.00%> (-0.07%)	⬇️
sourmash/lca/lca_db.py	97.17% <0.00%> (-0.05%)	⬇️
sourmash/cli/__init__.py	98.90% <0.00%> (-0.02%)	⬇️
... and 24 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2872aa5...26974e1. Read the comment docs.

[ctb]

hmm, may want to add conda-forge instructions to this. not sure how conda-forge handles updates!

[luizirber]

I added conda-forge instructions and announcement tweets for 3.3.0 and 3.4.0