chore: add PR template

[jacobowitz]

We decided in the Retro that we want more information in PRs as they also serve as a type of documentation. One tool of achieving this goal is the addition of a PR template.

This PR adds a template for PRs to our repo as described here.

The template now is super basic, it basically just asks you to provide a minimal description and to check if the PR is ready for review. In theory we could also have a more extensive checklist like "have you added tests", "did you read the contributor guide" etc. I think a checklist is not necessary at this point but might become in the future.

[codecov]

Codecov Report

Merging #4238 (0356fa8) into master (d1a0833) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4238      +/-   ##
==========================================
+ Coverage   87.16%   87.19%   +0.02%     
==========================================
  Files         113      113              
  Lines        8254     8254              
==========================================
+ Hits         7195     7197       +2     
+ Misses       1059     1057       -2     

Flag	Coverage Δ
daemon	42.23% <ø> (+0.03%)	⬆️
jina	86.93% <ø> (+1.90%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
jina/serve/networking.py	71.27% <0.00%> (-12.47%)	⬇️
jina/orchestrate/deployments/__init__.py	80.80% <0.00%> (-2.11%)	⬇️
jina/orchestrate/pods/jinad.py	93.02% <0.00%> (-0.78%)	⬇️
jina/jaml/__init__.py	95.43% <0.00%> (+0.38%)	⬆️
jina/helper.py	81.87% <0.00%> (+0.83%)	⬆️
jina/serve/runtimes/gateway/http/app.py	90.80% <0.00%> (+1.14%)	⬆️
jina/jaml/parsers/flow/v1.py	98.24% <0.00%> (+1.75%)	⬆️
jina/clients/base/http.py	96.22% <0.00%> (+1.88%)	⬆️
jina/jaml/helper.py	85.36% <0.00%> (+2.43%)	⬆️
jina/orchestrate/pods/__init__.py	90.00% <0.00%> (+3.12%)	⬆️
... and 4 more

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[github-actions]

Latency summary

Current PR yields:

• 🐢🐢 index QPS at 1014, delta to last 2 avg.: -34%
• 🐎🐎🐎🐎 query QPS at 46, delta to last 2 avg.: +47%
• 😶 avg flow time within 1.3462 seconds, delta to last 2 avg.: +5%
• 🐎🐎🐎🐎 import jina within 0.4098 seconds, delta to last 2 avg.: +14%

Breakdown

Version	Index QPS	Query QPS	Avg Flow Time (s)	Import Time (s)
current	1014	46	1.3462	0.4098
2.6.4	1549	29	1.0418	0.3583
2.6.3	1562	32	1.5066	0.3594

Backed by latency-tracking. Further commits will update this comment.