Add rationale.md to document design decisions that people may be curi… (#1627)

* Add rationale.md to document design decisions that people may be curious about later.

* Formatting

* OT -> OTel

docs/rationale.md

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+# OpenTelemetry Rationale
+
+When creating a library, often times designs and decisions are made that get lost over time. This
+document tries to collect information on design decisions to answer common questions that may come
+up when you explore the SDK.
+
+## Span not `Closeable`
+
+Because a `Span` has a lifecycle, where it is started and MUST be ended, it seems intuitive that a
+`Span` should implement `Closeable` or `AutoCloseable` to allow usage with Java try-with-resources
+construct. However, `Span`s are unique in that they must still be alive when handling exceptions,
+which try-with-resources does not allow. Take this example:
+
+```java
+Span span = tracer.spanBuilder("someWork").startSpan();
+try (Scope scope = tracer.withSpan(span)) {
+    // Do things.
+} catch (Exception ex) {
+    span.recordException(ex);
+} finally {
+    span.end();
+}
+```
+
+It would not be possible to call `recordException` if `span` was also using try-with-resources.
+Because this is a common usage for spans, we do not support try-with-resources.