all: update readme, rename to stellar-rpc, make sentry optional

[JakeUrban]

What

This PR contains changes made to wallet-backend necessary to get my local environment setup as well as changes to the README to provide additional guidance to future contributors.

Why

I had some trouble following the README as-is, and there were a few bugs / minor improvements worth making rather than working around them.

Known limitations

N/A

Issue that this PR addresses

Partially addresses #95

Checklist

PR Structure

• It is not possible to break this PR down into smaller PRs.
• This PR does not mix refactoring changes with feature changes.
• This PR's title starts with name of package that is most changed in the PR, or all if the changes are broad or impact many packages.

Thoroughness

• This PR adds tests for the new functionality or fixes.
• All updated queries have been tested (refer to this check if the data set returned by the updated query is expected to be same as the original one).

Release

• This is not a breaking change.
• This is ready to be tested in development.
• The new functionality is gated with a feature flag if this is not ready for production.

[aditya1702]

LGTM