updated readme

README.markdown

@@ -1,6 +1,26 @@
 # sneak/steemd docker image
 
-* wallet support disabled
+# Build Args
+
+    * `STEEMD_REPO`
+    * `STEEMD_REV`
+    * `SECP256K1_REPO`
+    * `SECP256K1_REV`
+    * `UBUNTU_MIRROR`
+
+# Environment Variables
+
+    * `STEEMD_WITNESS_NAME`
+    * `STEEMD_MINER_NAME`
+    * `STEEMD_PRIVATE_KEY`
+        * Bitcoin Wallet Import Format (WIF)
+    * `STEEMD_MINING_THREADS`
+        * default 8
+    * `STEEMD_EXTRA_OPTS`
+        * passed in to steemd on command line
+
+The init scripts will do all the silly escaping, just set these
+to the actual unquoted strings.
 
 # Versions