Define contract spec for Hash; adds instruction for XDR-gen (#361)

* Define contract spec for Hash

* Add instruction for xdr regeneration

README.md

@@ -88,3 +88,23 @@ stellar-xdr decode --type BucketEntry --input stream-framed --output json-format
 ```
 
 License: Apache-2.0
+
+
+### For Developers: How to Regenerate From XDR
+To regenerate types from XDR definitions
+1. Update XDR definitions
+```concole
+git submodule update --init --remote
+```
+The `--init` flag is only required for the first time setting up the local project. 
+`--remote` flag will make sure to fetch the latest changes from from the remote-tracking branches `curr` and `next` at [stellar/stellar-xdr]. 
+
+**_NOTE:_** if you had multiple remotes specified in the submodules (e.g. one tracking `stellar/stellar-xdr`, the other tracking `your-fork/stellar-xdr`),
+make sure the remote that tracks [stellar/stellar-xdr] match with what's specifies in the `.git/config` or `.gitsubmodules` (with `.git/config` taking precedence. If neither file specifies it, then `origin` is used).
+
+2. Recompile and test
+```console
+make
+```
+
+When the regenerated types are ready to be merged, make sure to commit the regenerated code file `src/curr/generated.rs`, `src/next/generated.rs`, the version string file `xdr/curr-version`, `xdr/next-version`, as well as the submodule files `xdr/curr`, `xdr/next`.