Merge branch 'readme' into 'master'

README: add plutus-ledger-api to required builds

See merge request !33

README.md

@@ -11,9 +11,18 @@ This package set has the following policy.
 * Non-versioned locations (git, unversioned tarballs) are permitted to allow
   GHC to advance.
 * The following packages must always build:
+  * plutus-ledger-api
   * plutus-tx
   * plutus-tx-plugin
 
+## Viewing Package Versions
+
+To see the full list of library versions in this repository, do.
+
+```
+nix flake show git+https://gitlab.horizon-haskell.net/package-sets/horizon-plutus
+```
+
 ## Updating the Package Set
 
 Generating the nix expressions for the package set is a two step process.