Adding RELEASING.md (#176)

* Adding RELEASING.md

* Updating wording clarifying release branch

* Changing release branch to main branch

RELEASING.md

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+How to release splunk-otel-ios:
+
+* Make sure to bump the release version on both SplunkRum.swift and Podfile
+* Build the main branch locally to ensure it builds correctly and all unit tests are passing.
+* Run splunk-otel-ios-crashreporting locally, making sure that the dependency points to the main branch to be released. If necessary, fix any breaking issues before releasing a new version of splunk-otel-ios. Follow the following steps with splunk-otel-ios-crashreporting as well.
+* Create a signed tag with `git tag -s X.X.X` filling in X with the next release version. Push this tag to the repo. You can also use Github's release flow, which will automatically sign the tag as well.
+* In github, go to the releases section on the right and click the Releases header. Then click “Draft a New Release.” Choose the tag you just created (or create a new one here) and fill in release notes.
+* Release the cocoapod. Follow the steps under 'Release': https://guides.cocoapods.org/making/making-a-cocoapod.html