Update README for new naming and document collision risk

- Update the Custom Units / Custom Dimensions examples to the new
  lowerCamelCase dimension properties (.length, .amount) so the docs
  don't demonstrate the now-deprecated PascalCase names.
- Spell out in the Custom Dimensions section that a Quantity is identified
  solely by its raw string across all linked modules, so two modules using
  the same raw value silently collapse into one dimension. Recommend
  prefixing custom raw values (e.g. "Acme.Money") to avoid collisions.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: rstacpoole

README.md

@@ -128,7 +128,7 @@ let registryBuilder = RegistryBuilder()
 registryBuilder.addUnit(
     name: "centifoot",
     symbol: "cft",
-    dimension: [.Length: 1],
+    dimension: [.length: 1],
     coefficient: 0.003048 // This is the conversion to meters
 )
 let registry = registryBuilder.registry()
@@ -159,13 +159,13 @@ let registryBuilder = RegistryBuilder()
 try registryBuilder.addUnit(
     name: "apple",
     symbol: "apple",
-    dimension: [.Amount: 1],
+    dimension: [.amount: 1],
     coefficient: 1
 )
 try registryBuilder.addUnit(
     name: "carton",
     symbol: "carton",
-    dimension: [.Amount: 1],
+    dimension: [.amount: 1],
     coefficient: 48
 )
 let registry = registryBuilder.registry()
@@ -183,7 +183,7 @@ We can extend this example to determine how many cartons a group of people can p
 try registryBuilder.addUnit(
     name: "person",
     symbol: "person",
-    dimension: [.Amount: 1],
+    dimension: [.amount: 1],
     coefficient: 1
 )
 let person = try Unit(fromSymbol: "person", registry: registryBuilder.registry())
@@ -196,14 +196,14 @@ print(weeklyCartons)  // Prints '350.0 carton/week'
 
 ### Custom Dimensions
 
-The built-in `Quantity` dimensions (`Length`, `Mass`, `Time`, etc.) cover the ISQ base
+The built-in `Quantity` dimensions (`.length`, `.mass`, `.time`, etc.) cover the ISQ base
 quantities, but you can define your own dimension when none of them fit — for example, a
 `Money` dimension for cost calculations. Because `Quantity` is `RawRepresentable`, you can
 add one with a static extension:
 
 ```swift
 extension Quantity {
-    static let money = Quantity(rawValue: "Money")
+    static let money = Quantity(rawValue: "Acme.Money")
 }
 ```
 
@@ -223,8 +223,16 @@ let volume = Measurement(value: 24, unit: .meter.pow(3))         // 24 m^3
 print(rate * volume) // Prints '4320.0 $'
 ```
 
-Unlike repurposing an unrelated base quantity such as `Amount`, a dedicated dimension can't
-silently cancel against unrelated units. Use a unique `rawValue` for each distinct dimension.
+A dedicated dimension is safer than repurposing an unrelated base quantity such as `.amount`,
+which would silently cancel against unrelated units that share it. But the `rawValue` namespace
+is **global**: a `Quantity` is identified solely by its raw string, across every module linked
+into the program. If two modules each define a dimension with the same raw value — say both use
+`"Money"` — they are treated as *the same dimension* and will silently cancel against each other,
+producing wrong results with no error. There is no central registry to detect the clash.
+
+To avoid this, namespace every custom `rawValue` with a prefix unique to your module or
+organization (e.g. `"Acme.Money"`, not `"Money"`), exactly as the example above does. Reserve
+bare names like `"Money"` for nothing, since you cannot know what another dependency has chosen.
 
 ## CLI