Update installation command to use sudo

[ManuelB]

Description

Update readme installation command to use sudo

$ npm install -g @codefuturist/email-mcp
npm WARN EBADENGINE Unsupported engine {
npm WARN EBADENGINE   package: '@codefuturist/email-mcp@0.2.1',
npm WARN EBADENGINE   required: { node: '>=24.0.0' },
npm WARN EBADENGINE   current: { node: 'v20.16.0', npm: '9.2.0' }
npm WARN EBADENGINE }
npm ERR! code EACCES
npm ERR! syscall mkdir
npm ERR! path /usr/local/lib/node_modules/@codefuturist
npm ERR! errno -13
npm ERR! Error: EACCES: permission denied, mkdir '/usr/local/lib/node_modules/@codefuturist'
npm ERR!     at async mkdir (node:internal/fs/promises:858:10)
npm ERR!     at async /usr/share/nodejs/@npmcli/arborist/lib/arborist/reify.js:521:20
npm ERR!     at async Promise.allSettled (index 0)
npm ERR!     at async [reifyPackages] (/usr/share/nodejs/@npmcli/arborist/lib/arborist/reify.js:234:11)
npm ERR!     at async Arborist.reify (/usr/share/nodejs/@npmcli/arborist/lib/arborist/reify.js:158:5)
npm ERR!     at async Install.exec (/usr/share/nodejs/npm/lib/commands/install.js:146:5)
npm ERR!     at async module.exports (/usr/share/nodejs/npm/lib/cli.js:133:5)
npm ERR!  Error: EACCES: permission denied, mkdir '/usr/local/lib/node_modules/@codefuturist'
npm ERR!     at async mkdir (node:internal/fs/promises:858:10)
npm ERR!     at async /usr/share/nodejs/@npmcli/arborist/lib/arborist/reify.js:521:20
npm ERR!     at async Promise.allSettled (index 0)
npm ERR!     at async [reifyPackages] (/usr/share/nodejs/@npmcli/arborist/lib/arborist/reify.js:234:11)
npm ERR!     at async Arborist.reify (/usr/share/nodejs/@npmcli/arborist/lib/arborist/reify.js:158:5)
npm ERR!     at async Install.exec (/usr/share/nodejs/npm/lib/commands/install.js:146:5)
npm ERR!     at async module.exports (/usr/share/nodejs/npm/lib/cli.js:133:5) {
npm ERR!   errno: -13,
npm ERR!   code: 'EACCES',
npm ERR!   syscall: 'mkdir',
npm ERR!   path: '/usr/local/lib/node_modules/@codefuturist'
npm ERR! }
npm ERR! 
npm ERR! The operation was rejected by your operating system.
npm ERR! It is likely you do not have the permissions to access this file as the current user
npm ERR! 
npm ERR! If you believe this might be a permissions issue, please double-check the
npm ERR! permissions of the file and its containing directories, or try running
npm ERR! the command again as root/Administrator.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactor (no functional changes)

Checklist

• My code follows the project's code style (Biome formatter + ESLint linter)
• I have run pnpm check and it passes
• I have run pnpm typecheck and it passes
• I have added tests that prove my fix/feature works (if applicable)
• I have updated documentation (if applicable)
• My changes generate no new warnings