Adding documentation for the proto2 incompatibility

[dmarticus]

I'd say this PR partially closes #162. TBH, today was my first day poking around with Skeuomorph, and I don't know enough about the existing design tradeoffs to know the extent of the incompatibilities between our library and the existing schema protocols. However, we do have one open issue (#218) that mentions the incompatibility between Proto3 and Proto2, and in lieu of changing the design of skeuomorph to support both protobuf protocols, adding the documentation around this behavior is a good first step in helping our users understand the intentional incompatibilities herein.

Super open to suggestions about what other known Skeuomorph incompatibilities exist, and would be happy to address them in this PR.

[codecov]

Codecov Report

Merging #321 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #321   +/-   ##
=======================================
  Coverage   83.93%   83.93%           
=======================================
  Files          29       29           
  Lines        1899     1899           
  Branches       25       23    -2     
=======================================
  Hits         1594     1594           
  Misses        305      305           

Impacted Files	Coverage Δ
...cala/higherkindness/skeuomorph/avro/Protocol.scala	31.81% <ø> (ø)
.../scala/higherkindness/skeuomorph/avro/schema.scala	57.62% <ø> (ø)
...cala/higherkindness/skeuomorph/openapi/print.scala	99.15% <100.00%> (ø)
...igherkindness/skeuomorph/protobuf/ParseProto.scala	96.42% <100.00%> (ø)

---

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 e46bd70...5eee2f3. Read the comment docs.

[pepegar]

LGTM, thanks @dmarticus !

[juanpedromoreno]

thanks @dmarticus !