Add .env.example file. Update Readme.

.env.example

@@ -0,0 +1 @@
+MAINNET_RPC_URL=<your RPC url>

README.md

@@ -11,16 +11,26 @@ This project uses NPM for dependency management and Forge for tests so you'll ne
 * Install NVM https://github.com/nvm-sh/nvm/blob/master/README.md#install--update-script
 
 * Install specific Node.js version
-    ```
+    ```sh
     nvm install
     ```
 
-* Installing the dependencies:
+* Install the dependencies:
     ```sh
     npm ci
     ```
+
 * Install Foundry and `forge` https://book.getfoundry.sh/getting-started/installation
 
+* Create `.env` file
+    ```sh
+    cp .env.example .env
+    ```
+
+    and specify there your `MAINNET_RPC_URL`.
+
+    > **_NOTE:_**  You may need to specify manually maximum allowed requests per second (rps) value for an API key/RPC url for some providers. In our experience max 100 rps will be enough to run tests.
+
 ## Running tests
 
 ```sh
@@ -30,13 +40,15 @@ forge test
 ## Test coverage HTML report generation
 
 1. Install `lcov` package in your OS
-    ```
+    ```sh
     brew install lcov
-    or
+    
+    -OR-
+
     apt-get install lcov
     ```
 2. Run
-    ```
+    ```sh
     npm run cov-report
     ```
-3. Open `./coverage-report/index.html` in you browser.
+3. Open `./coverage-report/index.html` in your browser.