sky130: documentation updates (#876)

hammer/technology/sky130/README.md

@@ -9,6 +9,8 @@ PDK Setup
 The Skywater 130nm PDK files are located in a repo called [skywater-pdk](https://github.com/google/skywater-pdk/). A tool called [Open-PDKs (open_pdks)](https://github.com/RTimothyEdwards/open_pdks/) was developed to generate all the files typically found in a PDK.
 Open-PDKs uses the contents in `skywater-pdk`, and outputs files to a directory called `sky130A`.
 
+This will take around 40-50GB of disk space as of September 2024.
+
 ### PDK Install
 
 ```shell
@@ -23,7 +25,9 @@ export PATH=$PREFIX/.conda-signoff/bin:$PATH
 
 # clone required repos
 git clone https://github.com/google/skywater-pdk.git
+# rev 7198cf647113f56041e02abf3eb623692820c5e1
 git clone https://github.com/RTimothyEdwards/open_pdks.git
+# rev 320597ea84b2816eb2fcc4fbe10c3874f19c92fc
 
 # install Sky130 PDK via Open-PDKs
 #    we disable some install steps to save time