Add Initialization message to WatchDocumentsResponse

[hackerwins]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line:

/kind api-change
/kind bug
/kind cleanup
/kind deprecation
/kind design
/kind documentation
/kind failing-test

/kind feature

What this PR does / why we need it:

When multiple users edit a document, the user wants to know who is connected.

Which issue(s) this PR fixes:

Address #48

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Additional documentation:

[codecov]

Codecov Report

Merging #64 into master will decrease coverage by 0.11%.
The diff coverage is 1.96%.

@@            Coverage Diff             @@
##           master      #64      +/-   ##
==========================================
- Coverage   54.03%   53.92%   -0.12%     
==========================================
  Files          23       23              
  Lines        2650     2663      +13     
==========================================
+ Hits         1432     1436       +4     
- Misses       1121     1130       +9     
  Partials       97       97              

Impacted Files	Coverage Δ
api/converter/from_pb.go	77.27% <0.00%> (ø)
api/converter/to_pb.go	73.01% <0.00%> (-3.66%)	⬇️
pkg/splay/splay.go	47.75% <ø> (ø)
yorkie/rpc/rpc_server.go	43.04% <0.00%> (-0.77%)	⬇️
client/client.go	57.48% <100.00%> (ø)
pkg/llrb/llrb.go	76.19% <0.00%> (+2.72%)	⬆️

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