docs: update compat matrix for boost 1.6.1rc

Author: Jacob Heun

getting-started/README.md

@@ -12,10 +12,9 @@ The Boost source code repository is hosted at [github.com/filecoin-project/boost
 
 | Boost Version          | Lotus Version                          |
 | ---------------------- | -------------------------------------- |
-| v1.4.0                 | v1.17.0, v1.17.1, v1.17.2, v1.18.0-rc1 |
 | v1.5.0                 | v1.18.0                                |
 | v1.5.1, v1.5.2, v1.5.3 | v1.18.0, v1.19.0                       |
-| v1.6.0                 | v1.20.0, v1.20.1                       |
+| v1.6.0, v1.6.1-rc1     | v1.20.x                                |
 
 ### Building and installing