Add contribution guidelines (#794)

* Copy in HowToWorkWithTheEpicsRepository.md from epics-docs
* Adjust content to remove base 3 information
* Add link from README

CONTRIBUTING.md

@@ -0,0 +1,41 @@
+# Contribution Guidelines
+
+For bug fixes, please create new pull requests against the default (7.0)
+branch. If the bug only affects a specific older release branch, target that
+branch instead.
+
+For new features, please open an issue or discuss the idea on mailing lists
+or in Matrix beforehand.
+
+## Maintaining Compatibility
+
+Build and test your changes on as many systems as possible.
+Important operating systems are Linux, Windows, macOS, vxWorks and RTEMS.
+
+## Testing
+
+All new features must come with automated tests to verify their correctness.
+This also helps to find out if future changes break existing features.
+
+EPICS Base comes with a testing framework which allows you to run IOCs,
+set and read/compare values and more.
+
+See any of the existing tests within the repository for guidance on test
+structure and dependencies.
+
+Add your tests to the appropriate test directory and ensure they are
+included in the test Makefile.
+
+Your test should run (and succeed) when you execute:
+
+```bash
+make runtests
+```
+
+CI will run these tests automatically. All checks must pass before a
+pull request can be merged.
+
+## Review Process
+
+The core developer team will review your changes, suggest changes, highlight
+issues or merge your code into EPICS Base.

README.md

@@ -54,9 +54,13 @@ Run the `dbl` command to list the records it provides:
 epics> dbl
 first:BaseVersion
 first:exit
-epics> 
+epics>
 ```
 
+## Contributing
+
+See [CONTRIBUTING.md](./CONTRIBUTING.md).
+
 ## License
 
 EPICS Base is distributed subject to a Software License