chore: add readme (#2)

Author: ethanndickson

.prettierrc

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+{
+  "printWidth": 120,
+  "semi": false,
+  "trailingComma": "all",
+  "overrides": [
+    {
+      "files": ["./*.md", "./**/*.md"],
+      "options": {
+        "printWidth": 80,
+        "proseWrap": "always"
+      }
+    }
+  ]
+}

LICENSE

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+The MIT License
+
+Copyright (c) 2025 Coder Technologies Inc.
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the "Software"), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+the Software, and to permit persons to whom the Software is furnished to do so,
+subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

README.md

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+# paralleltestctx
+
+A Go static analysis tool that warns when contexts with a deadline are used in
+parallel subtests.
+
+## Problem
+
+See [our blog post](https://coder.com/blog/go-testing-contexts-and-t-parallel)
+for an in-depth explanation of the problem.
+
+## Usage
+
+```bash
+go run github.com/coder/paralleltestctx/cmd/paralleltestctx@latest ./...
+```
+
+### Custom functions that produce contexts with timeouts
+
+By default, the linter detects `context.WithTimeout` and `context.WithDeadline`
+as producing contexts with timeouts or deadlines. Additional functions that
+create a context with a deadline or timeout can be specified using the
+`-custom-funcs` flag.
+
+```bash
+go run github.com/coder/paralleltestctx/cmd/paralleltestctx@latest -custom-funcs="testutil.Context" ./...
+```
+
+## Examples
+
+### ❌ Potentially flakey test
+
+```go
+func TestBad(t *testing.T) {
+   ctx, cancel := context.WithTimeout(context.Background(), 100*time.Millisecond)
+   defer cancel()
+   // WARNING: do not do this!
+   t.Parallel()
+
+   uut := newUnitUnderTest()
+   // Danger! Context may have timed out by this point
+   writeCtx(ctx, t, uut.Input, 5)
+   output := readCtx(ctx, t, uut.Output)
+   if output != 25 {
+      t.Errorf("expected 25 got %d", output)
+   }
+}
+```
+
+### ❌ Potentially flakey parallel subtest
+
+```go
+func TestBad(t *testing.T) {
+    t.Parallel()
+    ctx, cancel := context.WithTimeout(context.Background(), time.Second * 5)
+    t.Cleanup(cancel)
+    t.Run("sub", func(t *testing.T) {
+        t.Parallel()
+        // Danger! Context may have timed out by this point
+        doSomething(ctx) // Warning: timeout context used after t.Parallel call
+    })
+}
+```
+
+### ✅ Fixed test - create a new context
+
+```go
+func TestGood(t *testing.T) {
+    t.Parallel()
+    ctx, cancel := context.WithTimeout(context.Background(), time.Second * 5)
+    t.Cleanup(cancel)
+    t.Run("sub", func(t *testing.T) {
+        t.Parallel()
+        // We're shadowing the parent context with a new deadline, so this is fine
+        ctx, cancel := context.WithTimeout(context.Background(), time.Second * 5)
+        t.Cleanup(cancel)
+        doSomething(ctx)
+    })
+}
+```
+
+## License
+
+This project is licensed under the MIT License - see the [LICENSE](LICENSE) file
+for details.