Improve CONTRIBUTING.md (#3394)

* Improve "Working Directory" instructions in CONTRIBUTING.md (#1280)

The instruction wasn't clear to me and it took me some time to figure out what to insert into the "Working Directory" option.

Co-Authored-By: oonoo <2565796+oonoo@users.noreply.github.com>

* Improve "Arguments Passed On Launch" instructions in CONTRIBUTING.md (#1280)

I wasn't sure which arguments could be passed to SwiftLint. I referenced the Command Line paragraph in the README, it already lists the available arguments.

Co-Authored-By: oonoo <2565796+oonoo@users.noreply.github.com>

* Add more details to "Submodules" in CONTRIBUTING.md (#1280)

The reason to checkout git submodules wasn't mentioned. I added the use cases for submodules, as Swift Package Manager doesn't make use of the git submodules.

* Format "Using Xcode" in CONTRIBUTING.md (#1280)

Limit line length to 80 characters in "Using Xcode" section of CONTRIBUTING.md.

* Document changes in CHANGELOG.md (#1280)

* Add missing word

Co-authored-by: oonoo <2565796+oonoo@users.noreply.github.com>
Co-authored-by: JP Simard <jp@jpsim.com>

CHANGELOG.md

@@ -14,6 +14,10 @@
   [Keith Smiley](https://github.com/keith)
   [#3376](https://github.com/realm/SwiftLint/issues/3376)
 
+* Add more details to CONTRIBUTING.md  
+  [mknabbe](https://github.com/mknabbe)
+  [#1280](https://github.com/realm/SwiftLint/issues/1280)
+
 #### Bug Fixes
 
 * Fix parsing of Xcode 12 compiler logs for analyzer rules.  

CONTRIBUTING.md

@@ -17,8 +17,10 @@ immediately, but this should be the exception, not the norm._
 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. Set the "Arguments Passed On Launch" you want in the "Arguments" tab. See
+available arguments [in the README](https://github.com/realm/SwiftLint#command-line).
+1. Set the "Working Directory" in the "Options" tab to the path where you would like
+to execute SwiftLint. A folder that contains swift source files.
 1. Hit "Run"
 
 |Arguments|Options|
@@ -38,9 +40,11 @@ Then you can use the full power of Xcode/LLDB/Instruments to develop and debug y
 ### Submodules
 
 This SwiftLint repository uses submodules for its dependencies.
-This means that if you decide to fork this repository to contribute to SwiftLint,
-don't forget to checkout the submodules as well when cloning, by running
-`git submodule update --init --recursive` after cloning.
+The git submodules are used when using the Xcode project and workspace
+(e.g. `xed SwiftLint.xcworkspace`) and building SwiftLint by executing
+the `Makefile`. This means that if you decide to fork this repository to
+contribute to SwiftLint, don't forget to checkout the submodules as well when
+cloning, by running `git submodule update --init --recursive` after cloning.
 
 See more info [in the README](https://github.com/realm/SwiftLint#installation).