[MRG] simplify and clean up README

[ctb]

Many updates --

• remove HTML boilerplate at top of README
• remove mention of Rust from user-focused installation (adjust README to make it clear that bioconda installs everything #983)
• add cffi, numpy, matplotlib, scipy as documented requirements; remove ijson mention
• remove bioconda setup instructions since -c is now specified explicitly
• adjust language to point out that conda installs latest released version; link to releases page.
• adjust one line description at top
• adjust "who develops sourmash" a bit

Fixes #983
Addresses part of #1109

• 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?

[codecov]

Codecov Report

Merging #1124 into master will increase coverage by 9.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1124      +/-   ##
==========================================
+ Coverage   83.89%   93.04%   +9.14%     
==========================================
  Files          98       74      -24     
  Lines        9124     5810    -3314     
==========================================
- Hits         7655     5406    -2249     
+ Misses       1469      404    -1065     

Flag	Coverage Δ
#rusttests	?

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Impacted Files	Coverage Δ
src/core/src/wasm.rs
src/core/src/index/sbt/mod.rs
src/core/src/ffi/mod.rs
src/core/src/ffi/signature.rs
src/core/src/index/storage.rs
src/core/src/cmd.rs
src/core/src/sketch/minhash.rs
src/core/src/ffi/nodegraph.rs
src/core/src/index/linear.rs
src/core/src/sketch/nodegraph.rs
... and 14 more

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[ctb]

ready for review & merge @luizirber