Add module readiness checklist issue template

[clevinson]

Basic template from @aaronc's suggestions in #253

Some ideas of things that may be missing from here:

• do we need to audit anything about intermodule wiring in cases where that happens?
• I added a note about documentation (godoc + readme.md). We currently don't have the same docs structure as in the cosmos-sdk (where module directories have .md files with docs that get copied to their vuepress). What is our general documentation expectation for high level docs about things like functionlaity, store usage, API surface, events, etc.?
• Where in this checklist do we check for correct signature verifications? (e.g. that GetSIgners() is setup correctly for all Msg's), anything else abt authentication that we should add in this checklist?
• Do we need to add any base level x/upgrade support (now that we have consensusVersion, etc. from @technicallyty and @AmauryM's work?) as part of "Internal completeness audit" ?

[codecov]

Codecov Report

Merging #287 (a6025b4) into master (88e7ed4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #287   +/-   ##
=======================================
  Coverage   62.76%   62.76%           
=======================================
  Files          47       47           
  Lines        2968     2968           
=======================================
  Hits         1863     1863           
  Misses        893      893           
  Partials      212      212           

[aaronc]

ACK