Use pnpm as a package manager

[pnpm](https://github.com/pnpm/pnpm) is a package manager optimized for monorepos.
It is faster than NPM and consumes less disk storage than Yarn.

Disk consumption:
- Yarn
```sh
$ du -sh ~/workspace/acre/acre
2.2G    /Users/jakub/workspace/acre/acre
```
- pnpM
```sh
$  du -sh ~/workspace/acre/acre
875M    /Users/jakub/workspace/acre/acre
```

To install pnpm with Homebrew run `brew install pnpm`, for other installation options
please see the [documentation](https://pnpm.io/installation).

To install the packages dependencies run:
```sh
pnpm install
```

.gitignore

@@ -0,0 +1 @@
+node_modules/

README.md

@@ -6,6 +6,22 @@ Bitcoin Liquid Staking
 
 ## Development
 
+### pnpm
+
+This project uses [pnpm](https://pnpm.io/) as a package manager.
+
+#### Install
+
+To install pnpm with Homebrew run `brew install pnpm`, for other installation options
+please see the [documentation](https://pnpm.io/installation).
+
+#### Package Dependencies
+
+To install the packages dependencies run:
+```sh
+pnpm install
+```
+
 ### Pre-commit Hooks
 
 Developers are encouraged to use [pre-commit](https://pre-commit.com/) hooks to
@@ -34,3 +50,4 @@ pre-commit run --all-files
 # Execute hooks for specific files (e.g. Acre.sol):
 pre-commit run --files ./core/contracts/Acre.sol
 ```
+