[docs] Add draft PR instructions to PR template doc

[benraskin92]

What this PR does / why we need it:

Update template PR doc to include draft PR link.

Special notes for your reviewer:

Does this PR introduce a user-facing and/or backwards incompatible change?:

NONE

Does this PR require updating code package or user-facing documentation?:

NONE

[richardartoul]

LGTM

[codecov]

Codecov Report

Merging #1659 into master will increase coverage by 6.6%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1659     +/-   ##
========================================
+ Coverage    65.3%   71.9%   +6.6%     
========================================
  Files         958     964      +6     
  Lines       80131   80663    +532     
========================================
+ Hits        52369   58056   +5687     
+ Misses      24080   18805   -5275     
- Partials     3682    3802    +120

Flag	Coverage Δ
#aggregator	82.4% <ø> (-2.5%)	⬇️
#cluster	85.7% <ø> (+15.7%)	⬆️
#collector	63.9% <ø> (+8.7%)	⬆️
#dbnode	80.1% <ø> (+2.8%)	⬆️
#m3em	73.2% <ø> (ø)	⬆️
#m3ninx	74.1% <ø> (+5.3%)	⬆️
#m3nsch	51.1% <ø> (ø)	⬆️
#metrics	17.6% <ø> (ø)	⬆️
#msg	74.7% <ø> (ø)	⬆️
#query	66.3% <ø> (+19%)	⬆️
#x	86.3% <ø> (+8.8%)	⬆️

---

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 2ca0202...31e01fb. Read the comment docs.

[codecov]

Codecov Report

Merging #1659 into master will increase coverage by 6.6%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1659     +/-   ##
========================================
+ Coverage    65.3%   71.9%   +6.6%     
========================================
  Files         958     964      +6     
  Lines       80131   80663    +532     
========================================
+ Hits        52369   58056   +5687     
+ Misses      24080   18805   -5275     
- Partials     3682    3802    +120

Flag	Coverage Δ
#aggregator	82.4% <ø> (-2.5%)	⬇️
#cluster	85.7% <ø> (+15.7%)	⬆️
#collector	63.9% <ø> (+8.7%)	⬆️
#dbnode	80.1% <ø> (+2.8%)	⬆️
#m3em	73.2% <ø> (ø)	⬆️
#m3ninx	74.1% <ø> (+5.3%)	⬆️
#m3nsch	51.1% <ø> (ø)	⬆️
#metrics	17.6% <ø> (ø)	⬆️
#msg	74.7% <ø> (ø)	⬆️
#query	66.3% <ø> (+19%)	⬆️
#x	86.3% <ø> (+8.8%)	⬆️

---

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 2ca0202...31e01fb. Read the comment docs.