doc/source/install.rst: BuildStream depends on 'patch'

You will get errors like this if not:
"
Traceback (most recent call last):
  File "/root/.local/bin/bst", line 11, in <module>
    load_entry_point('BuildStream==0.1.dev1642+g8c0aaa1', 'console_scripts', 'bst')()
  File "/root/.local/lib/python3.6/site-packages/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "/root/.local/lib/python3.6/site-packages/buildstream/_frontend/main.py", line 132, in override_main
    standalone_mode=standalone_mode, **extra)
  File "/root/.local/lib/python3.6/site-packages/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/root/.local/lib/python3.6/site-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/root/.local/lib/python3.6/site-packages/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/root/.local/lib/python3.6/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/root/.local/lib/python3.6/site-packages/click/decorators.py", line 27, in new_func
    return f(get_current_context().obj, *args, **kwargs)
  File "/root/.local/lib/python3.6/site-packages/buildstream/_frontend/main.py", line 219, in build
    app.pipeline.initialize(use_remote_cache=True, inconsistent=track)
  File "/root/.local/lib/python3.6/site-packages/buildstream/_pipeline.py", line 142, in initialize
    self.preflight()
  File "/root/.local/lib/python3.6/site-packages/buildstream/_pipeline.py", line 155, in preflight
    plugin.preflight()
  File "/root/.local/lib/python3.6/site-packages/buildstream/plugins/sources/patch.py", line 61, in preflight
    self.host_patch = utils.get_host_tool("patch")
  File "/root/.local/lib/python3.6/site-packages/buildstream/utils.py", line 417, in get_host_tool
    raise ProgramNotFoundError("Did not find '{}' in PATH: {}".format(name, search_path))
buildstream.utils.ProgramNotFoundError: Did not find 'patch' in PATH: /root/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
"
diff --git a/doc/source/install.rst b/doc/source/install.rst
index ecd51b9..1836eea 100644
--- a/doc/source/install.rst
+++ b/doc/source/install.rst
@@ -41,7 +41,7 @@
 Install the dependencies with:
 
   sudo pacman -S python python-pip python-gobject git \
-                 ostree bubblewrap python-ruamel-yaml
+                 ostree bubblewrap python-ruamel-yaml patch
 
 Debian Stretch
 ~~~~~~~~~~~~~~
@@ -60,7 +60,7 @@
 
   sudo apt-get install \
       python3-dev python3-pip git python3-gi \
-      python3-ruamel.yaml bubblewrap
+      python3-ruamel.yaml bubblewrap patch
   sudo apt-get install -t stretch-backports \
       gir1.2-ostree-1.0 ostree
 
@@ -73,7 +73,7 @@
   sudo apt-get install \
       python3-dev python3-pip git \
       python3-gi gir1.2-ostree-1.0 ostree \
-      bubblewrap python3-ruamel.yaml
+      bubblewrap python3-ruamel.yaml patch
 
 
 Fedora
@@ -82,7 +82,7 @@
 requirements you need::
 
   dnf install -y bubblewrap fuse git python3-gobject \
-                 python3-psutil ostree
+                 python3-psutil ostree patch
 
 
 User installation with pip