Update substr.txt

[snakierten96]

Document the usage of $substr to cast an integer as a string

[tychoish]

Thanks for the contribution! First of all, sorry for the delay in getting back to you on this. I'd like to get this merged in an published, but first we need you to sign the MongoDB Contributor Agreement. Once we have the signature on file we can move forward.

Thanks so much for your effort, and sorry again for the delay!

Cheers,
sam

[kay-kim]

Thank you so much for your work on this.

We currently have a task of rewriting the documentation for aggregation, which includes standardizing the format of the aggregation operator pages (such as specification of the syntax and operands) and adding examples. https://jira.mongodb.org/browse/DOCS-2981. Since we will overhaul these pages, including the substr page, we'll close this pull request. But thank you for your time and pull request. Much appreciated.

Warmest Regards,

Kay Kim