Adding strings for dotnet-sln command for LOC

Adding strings both in the command folder as well as in the Help command.

src/dotnet/commands/dotnet-help/HelpCommand.cs

@@ -35,6 +35,7 @@ namespace Microsoft.DotNet.Tools.Help
   pack          {LocalizableStrings.PackDefinition}
   migrate       {LocalizableStrings.MigrateDefinition}
   clean         {LocalizableStrings.CleanDefinition}
+  sln           {LocalizableStrings.SlnDefinition}
 
 Project modification commands:
   add           Add items to the project

src/dotnet/commands/dotnet-help/LocalizableStrings.cs

@@ -61,5 +61,7 @@
         public const string VsTestDefinition = "Runs Microsoft Test Execution Command Line Tool.";
 
         public const string CleanDefinition = "Clean build output(s).";
+
+        public const string SlnDefinition = "Modify solution (SLN) files.";
     }
 }

src/dotnet/commands/dotnet-sln/LocalizableStrings.cs

@@ -0,0 +1,34 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+namespace Microsoft.DotNet.Tools.Sln
+{
+    internal class LocalizableStrings
+    {
+        public const string AppFullName = ".NET modify solution file(s) command";
+
+        public const string AppDescription = "Command to add, remove and list projects from the solution (SLN) file.";
+
+        public const string AppHelpText = "Projects to add to the solution.";
+
+        public const string CmdSlnFile = "<SLN_FILE>";
+
+        public const string CmdSlnFileText = "Solution file to operate on. If not specified, the command will search the current directory for one.";
+
+        public const string AddSubcommandHelpText = "Add a specified project to the solution.";
+
+        public const string RemoveSubcommandHelpText = "Remove the specified project from the solution. The project is not impacted.";
+        
+        public const string ListSubcommandHelpText = "List all projects in the solution.";
+
+        public const string CreateSubcommandHelpText = "Create a solution file.";
+
+        public const string MultipleSlnFilesError = "The current directory contains more than one solution file. Please specify the solution file to use.";
+
+        public const string ProjectFileNotFoundError = "The specified project {0} was not found.";
+
+        public const string SolutionFileNotFoundError = "The specified solution file {0} was not found.";
+
+        public const string NoSolutionFileError = "The current directory does not contain a solution file. Please specify a solution file to use.";
+    }
+}

test/dotnet-help.Tests/GivenThatIWantToShowHelpForDotnetCommand.cs

@@ -41,6 +41,7 @@ Commands:
   pack          Creates a NuGet package.
   migrate       Migrates a project.json based project to a msbuild based project.
   clean         Clean build output(s).
+  sln           Modify solution (SLN) files.
 
 Project modification commands:
   add           Add items to the project