Add README entries to explain nix-build (#679)

Author: sevanspowell

README.md

@@ -26,13 +26,15 @@ Possible networks: `etc`, `eth`, `mordor`, `testnet-internal`
 
 ### Building the client
 
-#### Prerequisites to build
+#### SBT
+
+##### Prerequisites to build
 
 - JDK 1.8 (download from [java.com](http://www.java.com))
 - sbt ([download sbt](http://www.scala-sbt.org/download.html))
 - python 2.7.15 (download from [python.org](https://www.python.org/downloads/))
 
-#### Build the client
+##### Build the client
 
 As an alternative to downloading the client build the client from source.
 
@@ -46,6 +48,39 @@ in the root of the project.
 
 This updates all submodules and creates a distribution zip in `~/target/universal/`.
 
+#### Nix
+
+In the root of the project:
+
+##### Build the client
+
+```
+nix-build
+```
+
+##### Regenerate lock files
+
+```
+nix-shell
+sbtix-gen-all2
+```
+
+###### Why so many lock files?
+
+- `repo.nix`                 : generated by the `sbtix-gen` command and includes only the build dependencies for the project.
+- `project/repo.nix`         : generated by the `sbtix-gen-all` command and includes only the plugin dependencies. Also generates `repo.nix`.
+- `project/project/repo.nix` : generated by the `sbtix-gen-all2` command and includes only the pluginplugin dependencies. Also generates `repo.nix` and `project/repo.nix`.
+
+##### error: unsupported argument 'submodules' to 'fetchGit'
+
+You get this error when you aren't using a new-enough version of Nix (fetchGit support for submodules is recent).
+
+To fix this, update the version of Nix you are using, or in a pinch:
+
+  - Remove the "submodules = true;" argument from fetchGit (in `./nix/pkgs/mantis/default.nix`).
+  - `git submodule update --recursive --init`
+  - `nix-build`
+
 ### Monitoring
 
 #### Locally build & run monitoring client