chore(docs): explaining the sheet crypto part

CONTRIBUTING.md

@@ -16,6 +16,21 @@ Add `client/.npmrc` file with following content:
 //pylon.sheetjs.com:54111/:_authToken="TOKEN-GOES-HERE"
 ```
 
+In addition to the registry auth, the `@sheet/crypto` package is installed from a local tarball
+(`client/libraries/sheet-crypto.tgz`) rather than the registry. This tarball is not committed to
+the repo directly - only an encrypted copy is: `client/libraries/sheet-crypto.tgz.gpg`.
+
+Before running `npm i`/`npm ci` in `client`, you must decrypt it using the passphrase
+(stored as the `SHEET_PWD` CI secret - ask a repo maintainer if you don't have it):
+```bash
+cd client
+echo "YOUR_PASSPHRASE" | gpg --batch --yes --passphrase-fd 0 \
+  --output ./libraries/sheet-crypto.tgz \
+  --decrypt ./libraries/sheet-crypto.tgz.gpg
+
+npm i
+```
+
 [Handsontable](https://www.npmjs.com/package/handsontable)
 
 Licence should be inserted in the `client/src/index.html` file:
@@ -66,6 +81,15 @@ To remove the release, you need to do it with repo administration over at [https
 
 # Troubleshooting
 
+## `npm i` fails with ENOENT on `sheet-crypto.tgz`
+
+If `npm i`/`npm ci` in `client` fails with:
+```
+npm error enoent ENOENT: no such file or directory, open '.../client/libraries/sheet-crypto.tgz'
+```
+it means the encrypted `sheet-crypto.tgz.gpg` hasn't been decrypted yet. See
+[Dependencies that requires licences](#dependencies-that-requires-licences) above for how to decrypt it.
+
 ## Makedata service "could not create directory" error
 
 The dcpath folder should have its permissions set so that the system account (SYSUSERID) can both read and write to it.