Add project documentation

AGENTS.md

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+All documentation, branch names, variable names, method names, and comments must use American English. Pull request titles and commit messages must also be in English.
+
+Do not create additional AGENTS.md files.
+
+Do not use the `--no-build` parameter when running the application or tests.
+
+All tests should use xUnit with the FluentAssertions package.
+
+Projects and tests must target .NET 8 by default and also support .NET 9, .NET 7, and .NET 6. Use the latest available C# language features when possible.
+
+Before adding, modifying, or removing methods or classes, review the README files from related repositories:
+- https://github.com/mlemanczyk/Recyclable.Collections/blob/main/README.md
+- https://github.com/mlemanczyk/Recyclable.Collections.Compatibility.List/blob/master/README.md
+- https://github.com/mlemanczyk/Recyclable.Collections.Concurrent/blob/master/README.md
+- https://github.com/mlemanczyk/Recyclable.Collections.Linq/blob/master/README.md
+- https://github.com/mlemanczyk/Recyclable.Collections.Searching/blob/master/README.md
+- https://github.com/mlemanczyk/Recyclable.Collections.TestData/blob/master/README.md

README.md

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 # Recyclable.Collections.Linq
-TBD
+
+`Recyclable.Collections.Linq` is an upcoming extension to the `Recyclable.Collections` family. It aims to provide memory-friendly equivalents of the standard LINQ methods so that recyclable collections can be used without unnecessary allocations.
+
+## Included
+* `RecyclableEnumerableExtensions` *(planned)*
+
+## Milestones
+1. 🅿️ Implement base enumeration utilities
+        1. 🅿️ `Where` and `Select` equivalents
+        1. 🅿️ `SelectMany` equivalent
+        1. 🅿️ `Aggregate` and related summarizing methods
+        1. 🅿️ `Any`, `All`, `Count` and `LongCount`
+1. 🅿️ Implement `RecyclableEnumerable` wrappers
+1. 🅿️ Add performance benchmarks
+1. 🅿️ Publish `0.0.1-alpha`
+
+## NuGet packages
+This repository is part of a family of NuGet packages:
+
+- [Recyclable.Collections](https://github.com/mlemanczyk/Recyclable.Collections/blob/main/README.md)
+- [Recyclable.Collections.Compatibility.List](https://github.com/mlemanczyk/Recyclable.Collections.Compatibility.List/blob/master/README.md)
+- [Recyclable.Collections.Concurrent](https://github.com/mlemanczyk/Recyclable.Collections.Concurrent/blob/master/README.md)
+- [Recyclable.Collections.Linq](https://github.com/mlemanczyk/Recyclable.Collections.Linq/blob/master/README.md)
+- [Recyclable.Collections.Searching](https://github.com/mlemanczyk/Recyclable.Collections.Searching/blob/master/README.md)
+- [Recyclable.Collections.TestData](https://github.com/mlemanczyk/Recyclable.Collections.TestData/blob/master/README.md)