Write docs/ Developing Custom Storages contents

[panarch]

[coveralls]

Pull Request Test Coverage Report for Build 4794428713

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 17 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.04%) to 98.875%

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Files with Coverage Reduction	New Missed Lines	%
core/src/data/value/convert.rs	3	99.78%
core/src/executor/aggregate/mod.rs	7	82.47%
core/src/executor/insert.rs	7	96.34%

Totals
Change from base Build 4777188706:	0.04%
Covered Lines:	43502
Relevant Lines:	43997

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💛 - Coveralls