Add postgresql install instructions for psycopg2 (#12)

## Notion ticket link

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justifications if necessary -->
## Implementation description
* Added instructions to the README to install postgresql if users
haven't already. You will need this for local development, as
encountered by @janealsh.


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expected results and include screenshots when appropriate -->
## Steps to test
1. try running instructions and see if `pdm run dev` works properly.

## Checklist
- [x] My PR name is descriptive and in imperative tense
- [x] My commit messages are descriptive and in imperative tense. My
commits are atomic and trivial commits are squashed or fixup'd into
non-trivial commits
- [x] I have run the appropriate linter(s)
- [x] I have requested a review from the PL, as well as other devs who
have background knowledge on this PR or who will be building on top of
this PR

Author: mslwang

README.md

@@ -64,18 +64,18 @@ git clone https://github.com/uwblueprint/llsc.git
 cd llsc
 ```
 
-- Create a .env file in the root directory based on the .env.sample file. Update
+- Create a .env file in `./backend` and `./frontend` based on the .env.sample file. Update
   the environment variables as needed. Consult the [Secrets](#secrets) section
   for detailed instructions.
 
 ```bash
 cp .env.sample .env
 ```
 
-- Build and start the Docker containers
+- Build and start the Docker containers (in detached mode)
 
 ```bash
-docker-compose up --build
+docker-compose up -d
 ```
 
 - Install pdm (this is a global installation, so location doesn't matter)
@@ -85,6 +85,14 @@ brew install pdm
 ```
 Otherwise, feel free to follow install instructions [here](https://pdm-project.org/latest/#installation)
 
+- Install postgresql (this is for local development)
+On macOS:
+```bash
+brew install postgresql
+```
+Otherwise, feel free to follow install instructions [here](https://www.postgresql.org/download/)
+Note: you will need this for `psycopg2`, which requires a config file that is part of postgresql.
+
 You will then need to go into each directory individually to install dependencies.
 
 FastAPI backend