build: add PR template #103

The Dapr CPP SDK repo should have a PR template for when end users create PRs contributing to the project.

Describe the solution you'd like
We should abide by a standardized PR template across all SDK repositories.

To implement, we should add a pull_request_template.md file. Something similar to what we have in dapr/dapr (https://github.com/dapr/dapr/blob/master/.github/pull_request_template.md), but also considering if we need special additions for SDK specific repositories. See Java SDK repo PR template for additional inspiration: https://github.com/dapr/java-sdk/blob/master/.github/pull_request_template.md

Additional context
This will help to improve contributors to easily communicate their changes, as well as maintainers to easily understand what the PR is doing.

Erstelle .github/pull_request_template.md für das Dapr C++ SDK. Vergleiche zuerst die in den Dapr- und Java-SDK-Repositories verknüpften Templates und nimm dann eine standardisierte Struktur mit allen SDK-spezifischen Ergänzungen auf; abgeschlossen ist die Aufgabe, wenn Contributors und Maintainer über ein klares, für das Repository geeignetes PR-Template verfügen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.