Add changelog w/ 0.18.0rc1 entries

CHANGELOG.md

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+## dbt 0.18.0 (Release TBD)
+
+## dbt 0.18.0rc1 (August 19, 2020)
+
+- Add "Referenced By" and "Depends On" sections for each node ([docs#106](https://github.com/fishtown-analytics/dbt-docs/pull/106))
+- Add Name, Description, Column, SQL, Tags filters to site search ([docs#108](https://github.com/fishtown-analytics/dbt-docs/pull/108))
+- Add relevance criteria to site search ([docs#113](https://github.com/fishtown-analytics/dbt-docs/pull/113))
+- Support new selector methods, intersection, and arbitrary parent/child depth in DAG selection syntax ([docs#118](https://github.com/fishtown-analytics/dbt-docs/pull/118))
+- Revise anonymous event tracking: simpler URL fuzzing; differentiate between Cloud-hosted and non-Cloud docs ([docs#121](https://github.com/fishtown-analytics/dbt-docs/pull/121))
+
+Contributors:
+- [@stephen8chang](https://github.com/stephen8chang) ([docs#106](https://github.com/fishtown-analytics/dbt-docs/pull/106), [docs#108](https://github.com/fishtown-analytics/dbt-docs/pull/108), [docs#113](https://github.com/fishtown-analytics/dbt-docs/pull/113))