docs: Add disclaimer to guide people to report issues in Lotus

[daviddias]

Merge once all the issues are moved to lotus and the issues feature is disabled.

[codecov-commenter]

Codecov Report

Merging #384 into master will decrease coverage by 0.19%.
The diff coverage is 20.00%.

@@            Coverage Diff             @@
##           master     #384      +/-   ##
==========================================
- Coverage   61.47%   61.29%   -0.18%     
==========================================
  Files          42       43       +1     
  Lines        2873     2867       -6     
==========================================
- Hits         1766     1757       -9     
- Misses        956      960       +4     
+ Partials      151      150       -1     

Impacted Files	Coverage Δ
storagemarket/impl/client.go	2.89% <0.00%> (-0.01%)	⬇️
storagemarket/impl/clientstates/client_fsm.go	100.00% <ø> (ø)
storagemarket/impl/provider.go	3.00% <0.00%> (-0.01%)	⬇️
...oragemarket/impl/providerstates/provider_states.go	84.72% <100.00%> (+0.07%)	⬆️
retrievalmarket/network/query_stream.go	66.67% <0.00%> (ø)
storagemarket/types.go	0.00% <0.00%> (ø)

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