[PATCH] README: add .env file to installation instructions

[Russell Currey]

Creating the .env file is mentioned in the installation documentation
but not in the README, so following only the steps mentioned there will
fail.  Add this and add a `cd patchwork` in there for good measure so
you could straight up copy paste the steps.

Signed-off-by: Russell Currey <ruscur at russell.cc>
---
 README.rst | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/README.rst b/README.rst
index 38c1847..b45c3e6 100644
--- a/README.rst
+++ b/README.rst
@@ -61,11 +61,16 @@ environment. To install Patchwork:
 
        $ git clone https://github.com/getpatchwork/patchwork.git
 
-3. Build the images. This will download over 200MB from the internet::
+3. Create a ``.env`` file in the root directory of the project and store your
+   ``UID`` attribute there::
+
+       $ cd patchwork && echo "UID=$UID" > .env
+
+4. Build the images. This will download over 200MB from the internet::
 
        $ docker-compose build
 
-4. Run `docker-compose up`::
+5. Run `docker-compose up`::
 
        $ docker-compose up
 
-- 
2.21.0