add build instructions to ota-provider-app

[holbrookt]

Problem

It is not obvious how to build ota-provider-app.

Change overview

Add build instructions to README.md

Testing

n/a

[github-actions]

Size increase report for "esp32-example-build" from 3a50501

File	Section	File	VM
chip-ipv6only-app.elf	.flash.text	-172	-172

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-all-clusters-app.elf and ./pull_artifact/chip-all-clusters-app.elf:

sections,vmsize,filesize

Comparing ./master_artifact/chip-bridge-app.elf and ./pull_artifact/chip-bridge-app.elf:

sections,vmsize,filesize

Comparing ./master_artifact/chip-shell.elf and ./pull_artifact/chip-shell.elf:

sections,vmsize,filesize

Comparing ./master_artifact/chip-ipv6only-app.elf and ./pull_artifact/chip-ipv6only-app.elf:

sections,vmsize,filesize
.flash.text,-172,-172
[Unmapped],0,-3924

Comparing ./master_artifact/chip-lock-app.elf and ./pull_artifact/chip-lock-app.elf:

sections,vmsize,filesize

Comparing ./master_artifact/chip-pigweed-app.elf and ./pull_artifact/chip-pigweed-app.elf:

sections,vmsize,filesize

Comparing ./master_artifact/chip-persistent-storage.elf and ./pull_artifact/chip-persistent-storage.elf:

sections,vmsize,filesize

Comparing ./master_artifact/chip-temperature-measurement-app.elf and ./pull_artifact/chip-temperature-measurement-app.elf:

sections,vmsize,filesize

[woody-apple]

Docs only, forcing this merge.