[CONTRIBUTING] Add building & running tips (#3360)

* [CONTRIBUTING] Add building & running tips

* fixup! [CONTRIBUTING] Add building & running tips

CONTRIBUTING.md

@@ -8,6 +8,33 @@ _If you have commit access to SwiftLint and believe your change to be trivial
 and not worth waiting for review, you may open a pull request and merge
 immediately, but this should be the exception, not the norm._
 
+### Building And Running Locally
+
+#### Using Xcode
+
+1. `git clone https://github.com/realm/SwiftLint.git`
+1. `cd SwiftLint`
+1. `xed .`
+1. Select the "swiftlint" scheme
+1. `cmd-opt-r` open the scheme options
+1. Set the "Arguments Passed On Launch" you want in the "Arguments" tab
+1. Set the "Working Directory" you want in the "Options" tab
+1. Hit "Run"
+
+|Arguments|Options|
+|-|-|
+|![image](https://user-images.githubusercontent.com/474794/93893073-1676b000-fca2-11ea-9428-20423c5f5237.png)|![image](https://user-images.githubusercontent.com/474794/93893135-27bfbc80-fca2-11ea-805a-ee21b446c3f0.png)|
+
+Then you can use the full power of Xcode/LLDB/Instruments to develop and debug your changes to SwiftLint.
+
+#### Using the command line
+
+1. `git clone https://github.com/realm/SwiftLint.git`
+1. `cd SwiftLint`
+1. `swift build [-c release]`
+1. Use the produced `swiftlint` binary from the command line, either by running `swift run [-c release] [swiftlint] [arguments]` or by invoking the binary directly at `.build/[release|debug]/swiftlint`
+1. [Optional] Attach LLDB: `lldb -- .build/[release|debug]/swiftlint [arguments]`
+
 ### Submodules
 
 This SwiftLint repository uses submodules for its dependencies.