Minor README updates

[unobe]

Changes proposed in this pull request:

• Update README to fix missing parenthesis
• Update README to add note for Mac OS users to run CouchDB

cc @HospitalRun/core-maintainers

[jkleinsc]

@unobe Thanks for the PR. I am concerned about adding the statement:

(Note: If on Mac, you need to make sure CouchDB can be run: go to System Preferences > Security & Privacy > Allow apps downloaded from: Anywhere)

Because this is a system wide setting, not one particular to CouchDB, it essentially is suggesting to a user that they should be lax about the security of their OS. I would prefer to recommend users just allow CouchDB specifically using instructions like the ones here under the section
How to open an app from a unidentified developer and exempt it from Gatekeeper:
https://support.apple.com/en-us/HT202491

[unobe]

@jkleinsc I've modified the patch to just refer to Apple's documentation: let the user understand what they want to do security-wise.

[jkleinsc]

@unobe Looks good to me. I'll merge it in. Thanks again!