Use relative positioning in the Podfile for "non-standard" pods

[mrose17]

Closes: #619

The basic idea is to clone the two "non-standard" repos at the same level as this repo and then use "../.." in the PodFile to reference them (rather than ~/...).

In addition to explaining that the two repos need to be cloned (and giving their locations),
the README.md file also gives the smallest number of steps necessary to get Xcode to build the project.

Unrelated rant: One might think that by now there'd be a file you could create, e.g., .xcenv that would define all the top-level values (bundle identifiers, team identifiers, etc.), so that developers could simply .gitignore the file and all the .pbxproj, .entitlements would be use ${...} syntax....

[ckrey]

@mrose17 I generally accepted your PR and suggestion, just changed the wording a bit

[ckrey]

Thanks

[mrose17]

Thank you!